In the Name of Tolerance, we will be intolerant!
Forward! ;-) In order to be tolerant and supportive of ... um ... "large people" we shall be intolerant of 4 year olds who ask frank and sometimes inappropriate questions. Hint to anyone who has never had a 4 year old - that just kind of goes with the territory. You can't sit down and go over every conceivable situation and question that might arise with a child before taking them out in public. This is how they learn. It is messy, funny, and sometimes embarrassing.
Maybe if the boy claimed he was transgendered or self-identified as female they would (re)welcome him with open arms? But if he pretended to shoot through the hole in a donut they'd probably ban him again.
In all seriousness though, I 100% support the right for businesses to refuse to serve anyone. (hint, that includes people who may offend or go against the religious sensibilities/beliefs of the business owner) In this case it seems like an enormous overreaction to ban the 4 year old. So while it is their right to refuse service, it is a stupid call on the manager's part.