I think I've talked them out of this:
So, I was playing around with another XXL t-shirt, when I discovered that this could be the perfect (!!!) body for Ruben's BAT costume:
Does my drawing make sense? The t-shirt is upside down. I put elastic through the bottom hem, just like in these skirts. I turned it inside out and stitched it along the dashed line. I cut two arm holes where you see those two vertical lines. His legs come out of what were once the t-shirt sleeves. This is so simple that it has already taken me much longer to write this post than it did to make the costume! This is just the beginning for our bat, but it would be a very cute body for many toddler-preschooler animal costumes.
It's really stretchy around the neck, which you need for easy on/off. He says it is very comfortable.
He gets into character as soon as he puts it on. I love that. I'll show you more pictures when he has wings!