I made several of these outfits with skirts and/or ruffles for Makayla and I sewed the buttons shut. I must say that Makayla is slender. She will be 3 in Jan and wears a 2T, is 24 pounds. Nicholas is a bit bigger, wears a 3t. All of the overalls and shortalls that have put on them slip right up over their diapers and all. They are cut larger than shorts or jeans, hence the suspenders. This is how I have done it, but I don't want to ruin your customs. Unless a child is really chubby in the tummy, overalls and shortalls should fit rather loosely. Now when I had 12 mo shortalls and made them into a jumper for Makayla in a size 2t, I cut above the buttons and attached a skirt. But you need the shortalls for the ruffles. If they are the right size for the girls, you should be fine. This is just my own way of doing it and I am by no means a designer! Do you know what size the girls wear and what size your shortalls are? Are the girls slender of chubby? Cameron is really chubby everywhere, but in his size 6-12 overalls they just slide right on up...they are made rather big, like I said and hence the suspenders to keep them up.
Let me know if I can help explain further....I know what I want to say, but I don't always make sense...arterial flow, you know!
Susan
ps if you look at that picture, you can see how large the shorts part is compared to the suspender and bib part. Like someone else said, you could leave the button open and make a casing with elastice at the sides that will then cover the buttons when they are closed, but that seems like a lot of work to me, and probably not necessary if you have the correct size shortalls.
good luck, you are doing a very nice thing...heading out to Joann's to get fabric to make one of those elf looking hoodies for Makayla and a patch European dress to match...and finishing Nick's pirate outfit for WDW trip 12/1....the vest is black with white skulls and red glitter eyes...he loves "spooky" things....can't wait for my first cruise...just got my travel documents. Gotta love the DVC and points...I will need Omar the tent maker to make clothes for me when I get back...right now I am Pooh size...but after a cruise and the
DDP, I should be tubby the tuba!
But here goes....
Some one hit the mother load. That looks like a lot of loot. have fun with all that. I can't waite to see what you make of all that.
purple and pink!
not at $47 he kept adding it on taking it off.. had to void it.. 
of course they are to tight on her thighs..
