I have seen this happen in a restaurant before. A mother had an uncontrollable toddler, and another mother had a happy toddler in a highchair. That was the only highchair. The mother of the uncontrollable toddler asked if she could use the highchair, and the other mother happily agreed.
What would I have done? If I was happy for my child to stay in the highchair, I would have answered no as well. However I don't necessarily think it's rude to ask at all. You won't know unless you ask.