Let’s get the simple idea out-of-the-way: Stop bringing the God damn toys to the playground.
With that done let me go into detail on the ‘why.’ See, we have children, lovely little people who are learning all the time, and sure it would be nice for all kids to know how to share when they were born. Truth being, my kid (at the time of writing) is not even two yet and does comprehend why he cannot edge his way into your kid’s space until your nervous five year old hands over the tractor in his sticky hands. This doesn’t teach him anything, sure my son will repeat “please” in his cutest voice knowing you – the parent (or nanny) – will hand it over. Then comes the grip. He’s young still, I know – and some concepts he grasps better than others, and sharing is not his power suit. Your kid wants to bring a toy? I understand more than you know – I’ve had to pry an ambulance from my son’s shocking capable grip – because: I don’t want to bring toys to the park. Because there might be younger kids there that haven’t learned to ask or share or *gasp* bullies who snatch up wind-up trucks for fun.
Just leave the toys at home and let the kids run around, find sticks, play on the swings on their own. Please.
(one caveat: if you child can handle the patience it takes, a ball is okay.)