The dreaded "W" sit.

There is some debate over whether that causes the hip/leg angles to be funny, or whether kids with funny hip angles find that a comfy way to sit.

Regardless, when SKnot started doing that, I was all over him to stop it.

As far as strengthening, go after the hip external rotators. They are the stabilizers for the whole kinetic chain of the leg. Throw in the abductors (pelvic/hip stabilizers), and you've got a lovely combo.