I wouldn't stretch in ABD. Also, how much can you trust an 11 year old? Perhaps it was that they felt their ego was bruised when you corrected them.
In standing, i keep an eye on their L/S to ensure it is in neutral, heel in line with femur and extension at the hip, not L/S or pelvis.
Some kids have or are going through a growth spurt so a chair to rest their knee on can help those with temporarily shortened quads.
Don't overstretch (of course!)
Can also do quads stretches in prone and sidelying for variations...
Have fun!