Help with Pants Mesh

Hi everyone! I'm new to meshing and I've been able to make a few simple things. I'm now trying to convert adult pants to child size. I'm having trouble with the waistband; some of the shirts stretch over the jeans, and the jeans go through most of the shirts. I've been working a while to try and play around with it, but I can't seem to get it right. Can someone help?

Thanks!

Blender File

Picture of the Jeans

(I hope I attached those right!)