Would it be legal to accept tips for copyrighted designs?

So I used to work at a theme park, where I did design based off of kid's handprints (like making them into a bird, dinosaur, etc.). Unlike the portrait artists, I was told I wasn't allowed to do these designs outside of work, since they are copyrighted. However, I was wondering if it would be acceptable legally, to take handprints and make designs from them (like at a street fair), but just accept tips for them? That way, I wouldn't be charging anything for them, and whoever wants to tip me can decide on how much. Would that still be against copyright laws?