I would come about it a slightly different way by saying to them (preferably in a group) that someone has been leaving chewing gum around which is a big hazard to your little one. You don't know who it is but it needs to go in the bin instead so that your baby doesn't choke.

That way it is clear it needs to stop but you aren't outing anyone or embarrassing them unnecessarily. If it still didn't stop then you can approach them directly.