Take some stretchy material and fold it in half.
Take a pair of your kids pants and fold them so the crotch is sticking out one side and there's a straight, outer-side-of-leg seam on the other side.
Line the outer side of leg sem up against the fold on your material.
Pin if you're feeling pedantic.
Cut around the shape of the pants adding about 2cm for seamage and about 6cm at top for hemmage.
Lay two cut pieces of material out flat.
Sew shorter sides together on both ides (crutch section)
Place crutch seams together (should now look like the shape of pants) and sew big U shape from one ankle, up to crutch and down to other ankle.
Add elastic in the waist hemmage space and you're done.
I assuage my too-slack-to-even-hem-the-ankles guilt by sewing a heart of the same material on the shirt to make a set.
Made a pair for Amy too but no cute pictures of them both as she is going through a need daddy by my side constantly stage leading to much heart wrenching tears when he is not around at bedtime (which face it, this is Japan, not having daddy here at bedtime is something she should be pretty used to by now...). So here we have Meg posing, K trying to get his aikido gear on- I'm assuming Aikido-ka do very few surprise attacks as it takes a good 10 minutes for K to gear up- and Amy wailing "Don't leave meeeee!!! I neeeeeeeed you!!!!!" The PJs are cute though, yeah?