My idea worked out quite well and went together pretty quickly, so here is my tutorial on how to make a pair of children's overalls from an old pair of adult pants. I think this project is probably for a medium level sewer.
Note: While denim is really durable, it also frays a lot in the wash. I recommend zig-zagging or serging everything as you go. You'll see the zig zags in my pictures. I think this project could also be quite nice made from some old khakis, or even corduroy.
Another note: I also recommend reading through all of the instructions at least once before you start.
I went through my denim stash and tried to find a pair of pants that wasn't too worn in the knees or on the bottom. Using a pair of Beeper's pants as a guide, I cut out the leg pieces. You can see I used the side seam and hem from the old pants to save myself some sewing. Don't forget to leave a seam allowance for the crotch and inside leg seam (I did 1/2”) and 1” at the top for the waistband.
For a normal pair of pants, the front and back pieces would be cut a little bit differently at the crotch. Otherwise, they fit funny. For overalls, I figured it didn't matter as much.
Fold each leg piece with the right sides together and sew the inside leg seams.
Turn one leg right-side-out and stuff it inside the other, matching up the crotch. Sew the two together along the crotch.
Use a piece of scratch paper held up to your child to decide how big you want the “bib.” Make a pattern, then cut two of that piece. Don't forget to give yourself seam allowances and an extra 1” on the bottom. If you want, you could cut one of these pieces from a part of the pants with a pocket so you'll have a pocket on the front of your overalls. I didn't think of this at the time and have had a request for a pocket since, so now I will have to cut a pocket off the pants and sew it onto the overalls.
Sew the two bib pieces together, with right sides together, leaving the bottom open. Clip the corners.
Turn bib right-side-out. Sewing around the edges at this point will make it look really nice. (Some people, who aren't as lazy as me, would even press it first.)
Center the bib on the front (outside) of the overalls (unsewn edge should be matched up to the waist of the pants) and pin. Sew 1” down from the top of the pants. (I took pictures of the last few steps, but they're missing now. :( Sorry.)
Zig-zag around the top of the pants, then fold down 1” and pin. Sew this down all around, trying to stay about 1/4” from the edge (not the fold).
At this point you need to try it on your child to find out how long you will need the straps to be. Depending on the child, this may be the trickiest part. While you have it on him/her, you may also want to figure out how much gather you want in the back, if you plan to do that part (see below).
I cut out the waist band from my old pants (and cut off all the belt loops) to use for straps. This is where I had to use a second pair of pants for the second strap. You might be able to squeeze two straps out of one pair of pants, depending on the size of the child and the pants. I had been thinking it would be nice to just use the button that was already there, but there was a rivet in the way when I tried to cut it out, so I had to take my strap from the middle of the waist band instead.
Since it came from the middle, that also meant that I had to make the ends nice. If you have a serger, you could probably get away with serging along all of the ends. I don't, so instead I had to unpick a few stitches, tuck the ends inside, and sew them up.
Now you'll pin the straps onto the back of the overalls where you want them. Sew along the waist line seam and as close to the fold as you can.
This next part is optional. It gives the back a little bit of gather, which gives the whole thing a little nicer shape. I cut a little slit in the waist on either side for threading the elastic through:
I took the piece of elastic I had measured out, threaded it through the back, and sewed it in. Then I zig-zagged back over the holes I had cut.
Now all that's left is to add buttons and button holes. Or you could use heavy duty/upholstery snaps, which is what I did. I think they are easier and sturdier. They look really professional too.
Now just try it on your child, and viola! (Actually, you may want to try it on before you do the buttons, just to make sure everything fits well.)
Then try to get him to stand stand still for a picture.
Beeper loves his overalls! He begged all day yesterday to let him wear them, but I told him he has to wait until Thursday. I may have to make him a couple more pairs. (And this is the same kid who hated overalls as a toddler.)