This happens often at the park when we are playing with a ball on one of the mini basketball courts. Even if we only have 1 ball and every other court is wide open, parents of toddlers will try to force us to give their baby a turn. I tell them "This is a large, hard basketball and it will hurt if it hits your child. Plus, these kids are using their ball and playing a game. They are not waching out for babies."
We have had moms stand by their child after stealing a ball and not give the ball back or apologize. One M. even kept telling us "He wants your ball. He saw your ball and doesn't have one here so he wants your ball." I told her "We are using the ball and he is too young to safely play with us."
If we use swings and there are several others open kids will sometimes come over and ask to use the two swingswe are taking turns on. I point to the adjacent swings and to the swings on the other side of the playground and say "Hey, there are some swings open right there. We are using these."
I was pretty surprised the first couple of times, but now I am annoyed at the "entitlement" attitude as someone said and the lazy parenting. If my child threw a fit at the park, we would leave and not come back for a long time. I would never go ask someone to give up their toy to my child. I would also not encourage my child to stand behind swings being used when there are 3 or more right there not being used.
One 8 or 9 year old girl kept following us and as soon as my child got on something she would say "I would like a turn on that now." We kept leaving and going to different stations. When I began pushing my child on one of the tire swings, she asked within maybe 20 seconds. I said "We just got on here and when she is finished with her turn, you are next." When we finished, about 4 minutes later, I said "Now it is your turn." We walked off and she did not get on the swing. I told my daughter to follow me. We got a drink and went back to the tire swing and she came back again. I told her it was our turn again now and it was going to be a long one. She left. Her M. was right there hearing and seeing everything. I used to push kids whose moms were too lazy to push them, but it took away from my child so I no longer do this. Plus, it helps enable the bad parents to not interact with their own children.
*I am all for sharing in appropriate situations and we do sometimes invite kids to join in and play with us as long as they follow the rules and play safely. I do not allow kids to come over and start keeping the ball from my or my friend's kids as it is our ball.