Playground Etiquette

Related to the topic below, we've had a few incidents at local playgrounds lately that I'm not sure how to handle.  My husband and I have been discussing it, but input from others would be great.  We currently go to playgrounds daily, sometimes twice a day if it's a cool day.  A few times Sam has been pushed out of the way by an older child who wants to play with the same toy.  Usually the caretaker swoops in and deals with the situation appropriately, but on occasion mom is busy talking, watching a sibling, or no where in sight.

Last week we were in a busy Davis Square playground with our stroller parked next to a picnic bench while Sam took a water break.  A little boy who looked like he was about three came by and stroller-jacked our stroller.  His mom happened to be sitting at the table.  At first he just did circles and I let it go since we were right there.  Then he started to walk away with it, with my diaper bag, phones, keys, etc, in it.  Sam started to get upset reaching for it and I was waiting for mom to ask him to stop.  She carried on her conversation ignoring the situation.  I asked him politely to return with it, he kept going.  I put Sam down and went over to him and asked him again.  By now he was far enough away that I had was not comfortable with him going any further, or with leaving Sam.  Mom continued talking and Sam is now about to cry, the kid is breaking into a run.  Finally I reached out took the stroller back and told him it was not his.  He stomped his feet at me, said something unintelligible and ran off.

Yesterday at another playground, two little boys from the same family physically moved Sam to get into Little Tike cars that he was using.  One slid in behind him, pushing him, as he was climbing into it.  Mom was on her cell phone turned away.  The first time I told the older brother that Sam had been using it and he had to wait his turn.  He looked at me, said, "no," and took off.  The second time Sam started wailing and the little boy also took off in it.  Other moms and dads saw the situation, but not cell-phone mom.  My husband told me I should have taken the boy out of the car, and explained it was not his turn, which I do not feel comfortable doing.  I'm trying to decide how I would feel if an adult removed Sam from something?

I'm a teacher, I deal with behavior all day long.  I give consequences, sort out what is "fair," and give positive reinforcement.  I try to do the same with my son.  After these things happen, I explain to Sam that the other children are not doing the right thing, but all he knows is they have what he was playing with and wanted.

I know at some point (soon as he is starting group daycare in a few weeks), he will come across this a lot more and for the rest of his life.  Meanwhile, how do I deal with non-observant parents and children who aren't behaving?