@MMarciano,
I've myself, gone out in the woods before - camping at night as I didn't want to walk all the way to the bathrooms that late.
Common sense tells you - you are a highway/road where this is nothing for miles and miles - kid needs to pee. You pull over, go a little ways off the road for privacy and pee. Not a big deal.
But peeing right there in the open where everyone is - but yes the store should have accomodated the kid.
I do have a funny story about a similar thing. My former hair dresser told me - they were at the beach with her young son. He came up to her and said he had to pee. It was quite a long walk to the any restroom so she told him quietly - just go pee in the ocean. So he goes walks up to the surf (and rather than walking in and peeing), he proceeds to pull his pants down and pee in the surf.