J.S.
I wouldn't let the kids go on anywhere that could possibly be considered public or private property not belonging to you. There are strict sanitation codes and if you got caught you could be hit with some hefty fines. Just saying.
When my girls do this I make them wait until we reach where we're going as long as it's within 10 or 15 minutes. If it's going to be a long ride then we stop at a Dunkin Donuts to use the bathroom and then I get myself a drink as a reward. :-) My girls are prone to UTI's and bladder infections and my eldest gets kidney reflux so I have reason not to make them wait for long. It's just not healthy.