How do I say this

[krysalyss]

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.

[Ca Plane Pour Moi]

Before you say the gum leaving is all about your DS, consider this:

Would you still want change without your DS about? (I would, who wants dirty used gum around anyway? I wouldn't even mention DS as it isn't acceptable without a small child around either.)
How do the boys feel about DS? Do they visit to see him (so good, behaviour needs to change because of DS) or do they not really care about DS (we were here first/don't see surprised as much cos of DS - so don't blame him for this!)