I got size 16R for a 14yr old who is fairly tall and they were way to short. All kids differ in height, so here's the measurements for size 16R: Inseam roughly 26", Waist 14.5 inches when laid flat (so I guess that means 29" waist). The measurement from the top of the jeans waist down to the leg is 36inches. So, I would take your kids jeans, see how they compare to these and then order up or down as needed.

I really like the jeans. The indigo color is very nice and the material is more on the light-weight side and nice and soft. No need to break these in. Comfy right from the start.
~ We'll be checking out larger sizes.