The kids want to be pushed on the roundabout at the park, so I'm going to push it so fast that they can't get off.
That is a little uncalled for to take things to that extreme.
Maybe, but I want to see them get scared
I think you need to reconsider what you're doing. Parents shouldn't be deliberately scaring their children for no reason. Find a more wholesome way to have fun with them!
I just want to have fun. They are just little kids, they won't remember it in a while.
Some memories from childhood can be remembered for a long time, especially if they were particularly traumatic. You should find ways to have fun with your children without deliberately scaring them.