Here's my thoughts, and this is coming from a mom who does try very hard to watch her children when they play, but also does not believe in "hovering" and lets them have their space; also speaking from the perspective that no matter how much I teach and try, my 4 year old is still very spirited and doesn't always behave the way he should.
I try to watch him closely, but with a 17 month old as well, I know not everything done by my 4 year old is seen by me. So here is how I would handle it if someone was threatening harm to my youngest, or how I would be comfortable with some stranger dealing with my 4 year old if I missed it.
I would let go of the sucker issue. I agree it's rude, and I would not let my children do that, but unless there is a posted rule against food and drink (in which case you could just report it to security or whoever manages the play area), then just let it go and wash your kid down real good when you leave to wash off the sticky. As far as kids doing things and your son imitating, I think all you can do is teach your son that's not the best way to play. My 4 year old will now ask, why can't I jump off, they did? And I just tell him the answer, "You can't do everything the other kids do. I said no and you need to listen to me. There are babies that could get hurt and I don't want you to jump." This way, your son learns why it's not appropriate, and if some other parents simply "overhear" your discussion with your son, maybe it will draw their attention and they will tell their kids to stop too. If the parents know their kid is doing it, and they just don't care, asking them to watch their children or asking them to tell their child to stop won't do any good.
I personally don't feel comfortable approaching another parent. I think it's very appropriate to say to another child who has injured or nearly injured your child, "no, no, be careful. He's a baby and you nearly jumped on him." or "wait a minute, don't go down the slide yet until the baby gets out of the way", or "no, honey, don't run out there, stay in here with your daddy". Especially if your words are firm, but kind, the other child learns a valuable lesson, and one of two things happens with the parents. If they simply weren't paying attention, then your words may draw their attention and they may start parenting their own child appropriately. Or, if they just don't care, than you won't offend a parent for "parenting" their child because you were kind in how you approached it, but the child will still learn how to play appropriately (since they are clearly not learning it at home). I know if I heard another parent say something like that to my older son, I would immmediately be alerted to find out what happened and have the opportunity to apologize and deal with my son in whatever way needs to be done.