I don't see a problem with the pee sitting in the tank, but if the pee makes it to the discharge valve there could be a problem, but even then I doubt it as there should not be much volume out of a 4 year old. As a precaution, I would put a bucket under the discharge valve and pull the handle. That way you make sure the pee is in the tank instead of backed up against the valve.
Chap , DW Joy, and Fur Baby Sango
2006 Ford F150 Super Cab 4x4
2008 Surveyor 263