If your little girl is too short to reach the toilet where else can she pee at a public restroom?

There are many solutions. I recommend that you both go into the wheelchair stall. Then let her pee into the trash can. She can stand with her legs spread apart over it or she can sit on it. I used to do it when I was little, and it's really easy to reach and actually quite hygienic.

You can also purchase a bottle* for her and she can pee-pee into that. Then just dump it into the toilet.

* A good brand of bottle is My Pee-Pee Bottle, but a water bottle would do fine - if she's a good "aimer". To help her, tell her pee a little (some might get on the floor) and then adjust the bottle to capture the urine.