"We have to go soon, we have enough time for you to go down the slide about 10 more times."

Both you and the OP are right. Phrasing like this example though should help. This way it's not you restricting the kid, and there's no hard requirement.

If they try to drag it out, you tell the kid that they had the time; it was their choice to take longer than usual to go down the slide those last few times.

That explaination also helps them realize that they can choose to either make the best of a situation or the worst of a situation, but the situation will happen regardless (the time limit).