Strap length for a kids’ shoulder bag... ❓

Hi everyone, 🙋
I’m currently pretty stumped about the strap length for a crossbody bag I crocheted for an 8-year-old girl. In my opinion, she’s “average” height—neither very tall nor very short. Unfortunately, I can’t measure her or have her try it on, which makes it really hard for me to decide. Maybe one of you has an idea or can at least give me an approximate measurement in ❓ cm—that would really help me out and be enough, since I have absolutely no idea myself.
Hoping for a reply, warm regards to everyone reading this. 🥰
Best, Channa

I’d look at a knitting pattern suitable for that age, for a jacket or sweater. Then you’ll have the one-way strap length from the shoulder to the hip.