For the arm measurement, Measure from A: just distal to the acromion to the B: lateral epicondyle to the C: metacarpal phalangeal (MCP) joints.
The leg measurement is on the outside of the leg (not the inseam), from the D: greater trochanter to the E: knee joint line to the F: floor—with the shoe on. Measure with the shoes/AFOs that that rider will wear when riding.
Measure both arms and also both legs, there may be a difference. If there is, use the shorter measurement. If there is a great deal of difference, call for adaptive accessory advice 800-838-1845 x117.