(If you didn't know :-) Both of my younger children have profound disabilities. They will both always be in diapers and they both will dig in their diapers. Therefore, they both must wear a onesie. There are special needs clothing websites that you can order from, and before I started sewing, I did order from them, then I realized that they are really not that difficult to sew. There are no commercially available patterns for a onesie style bodyshirt once you get above about a size 3T or 4T (that I am aware of), so I just sort of winged it.
I'll walk you through the steps, but do keep in mind that you should alter my calculations to fit YOUR child. I like my kids bodyshirts to have A LOT of room for growth because, well, frankly, I would much rather be sewing something other than onesies :-). If you prefer a snug fitting crotch area, try taking off some of the length of the back crotch extension (take off no more than 1/2 inch at the time or you will wind up too short!!)
**Ok... the first thing you will need to do is start with a standard tshirt pattern. My favorite pattern company is Ottobre (a Euro pattern company), but I know many home sewists use one of the big 3 or 4. (McCall's, Butterick, Simplicity, but I think of all the readily available US patterns, Kwik Sew is the best, I really can't suggest a specific pattern)
**Next, you will need something to trace your pattern on. Many sewists may use carbon paper and the little wheel thingy to copy the pattern onto the fabic... not this time! LOL Get yourself something called Pattern Ease (or Easy Pattern). They changed the name and I'm not sure what it is. Anyhoo- it is available at Joann's. Alternately, you can use Swedish Tracing Paper, doctor's office tissue paper, etc.. something that you can see the lines of the pattern through).
Leave off the hem on the bottom of the pattern piece. Typically that is going to be 1/2 inch to 3/4 inch of the length on the bottom edge of the pattern piece. (In other words, if on your pattern piece for both the front and back pieces there are 2 lines on the bottom, one for where to cut the fabric and one for where the hem is to be turned under, you want to only trace the upper most hem line. Let that be the bottom of your pattern piece.) If you find that it is still a little too long for your liking, you can try tracing the hem of the next smaller size. (ie- trace out a size 12 shirt, but when you get to the bottom/hem area, trace the hem of the sz 10)
**Now you need to measure your child. Use a tape measure and with your child laying on the their back, place the end on your child's shoulder. On the flat part of their shoulder, in the middle. Measure down to the middle of their crotch (on a diagonal).
#1-Trace your (store bought) pattern just as you would if you were going to do the tshirt.
#2- Now place your tracing of the front piece on the edge of your tracing paper (tape it in place)
#3- Use your tape measure, place the end on the shoulder area (just as you did on your child) and measure on the diagonal to the edge of the paper. You want the number of inches that you just measured on your child (this shirt is for my son. He measures 28" on the front). Mark your paper.
#4-How wide do you need the crotch? Measure across the crotch. For my son (size 14, I use 4.5", and for my daughter, sz 4, I use 3.5") Now take 1/2 of that measurement. That is how wide you need to draw the width of the crotch. (so for my son, with a 4.5 wide crotch piece, I will measure out 2.25" for the width on the pattern piece)
#5-Now just draw a curved line to connect the shirt hem to the crotch line. It doesn't have to be perfect or pretty! Just sketch it out lightly until it looks right to you.
#6- Now add two 1" sections to the bottom (as seen in red) for a total of 2 inches. (These 2 " will be folded up and add stability for your snaps)
Now for the back piece..........
#7- Take your pattern piece for the back of your shirt and place on the straight edge of the tracing paper and tape.
#8- Now take the torso measurement from the front and add ease to it. Here is "my" chart... right or wrong... this is the numbers I use. Feel free to adjust to your needs. sz 4-5= add 5" sz 6-7= add 5" sz 8-9= add 5" to 6" sz 10-12= add 6" sz 12-14= add 6"
**** Depending on where you would like your snaps, there is an alternative method. If you want the snaps further down between the legs (ie- my son has been undoing his lately, so I now place the snaps a bit further between the crotch v/s sitting at-in front of the crotch) So what I now do is take the above numbers, divide by 2 and add that number on to both the front and back length. So instead of adding 6" onto the back length. I now add 3" on front and 3" onto the back lengths.
#9- Using your tape measure, place one end at the shoulder, just as you did for the front piece. Measure diagonally the length that you get when you add your torso measurement + ease. For my son, a sz 14 with a 28" torso, I get......... 28" + 5"= 33" back length (I should have used 6"). Mark that measurement on your paper.
10-Make the crotch width the same as you did for the front.
11- Now you will want to connect the hem of the shirt to the crotch width area. You can see that for the front, I curved the line inward (concave) but now for the back, you'll want to curve it outward (convex) to cover their bottom.
12- About 3/4 of the way to the bottom is where you want to bring the rounded out bottom toward the crotch and make the rest of it the same width as the crotch piece. I know it sounds confusing, but just try to copy what I have done in the pic LOL! Again, it doesn't have to be perfect or beautiful! It'll come with experience.
13- Add two 1" sections to the bottom of the crotch area (total of 2" long by crotch width wide) as seen in red. These will be folded under and will help support your snaps.
14- You don't need to alter your sleeve pattern piece at all. Seen here is a long sleeve pattern piece, but I made it short sleeves for the summer.
15- Now trace out garment onto fabric, adding seam allowance if needed.
16-Sew garment together just as you would a tshirt, except for the hem, of course.
17- I finish my onesies with ribbing at the legs. You could also fold under the edges and coverstitch, or serge/jig jag and fold under and topstitch. I use ribbing because we were always popping the stitches of the turned under hems.
18- After you finish the legs (either ribbing or hem), fold under one 1" section of the front piece crotch (shown in red). Press. (I also use glue stick, works wonderfully!!) Fold under again to the next (red) line. Press and pin.
19- Using a straight stitch, sew across the top and bottom of this 1' area.
20- Repeat step 18 & 19 to the back shirt piece.
21- Apply snap tape to the "right" side/ top of the front crotch piece.
22- Apply opposite side of snap tape to the "wrong" side of the back crotch piece.
In this shirt below, I did not use snap tape. I used resin snaps. I usually use snap tape and just wanted to give this a try. FYI- before I folded under the 1" crotch sections, I put some twill in there to help support the resin snaps so they wouldn't pull though.