The hip adduction ROM test (side lying) and abduction is so clumsy for me. In the “Measurement of Joint Motion - A Guide to Goniometry” they teach doing these tests supine to keep the pelvis stable - with the proximal arm of the goniometer over the ASIS’s. Is there any specific reason why you would have us do this test differently? 
use what is most convenient. Sidelying requires some effort on part of tester. Since you are small doing test on floor or with a lowered table may be a compromise. If not then use supine test you found. namaste m