dear lola,
The two ways of testing muscle length are the active range of motion (AROM) and the passive range of motion (PROM).
In young children we would test when spasticity or a joint problem is suspected.
In the case of suspected spasticity, the hip adductors are tested passively in supine position with hips and knees flexed approx. 60degrees (Tardieu) and the hips are slowly abducted.
In the case of a suspected joint problem, I would also test in supine and 90degrees flexion hips
As far as I am informed there are still no standard values for the ROM in childrens joints. A baby of 9 mo old is even more challenging.
Good luck
I suggest you observe some children of that age and see what they are doing, it might give you some ideas!
kind regards and good luck with your studies
Esther