Although I can understand a parent who happens to drive past wanting to watch their child play for a few mins yet not disturb the child's routine, it's the behind the tree which is beyond odd.

I'd have waved - that would have been clear that I had seen them. I don't like sneakiness.

If they wanted to watch, fine, sit in the car and do it, no need to lurk in bushes and potential scare the children.