The scenario: A kid in the park in front of the water fountain and he's misusing it and leaving it open so an old man approaches him and says : could you close the tap please .. this is not your dad's water .... Which means he should save water because it's for everybody and the kid is acting as if he's in his own house . So what sarcastic idiom in English can be used instead of "this is not your dad's water"???
There's really no direct idiom for this specific scenario. There are two related sarcastic English idiomatic expressions, though.
A: Could you clean those dishes off the table for me (or some other chore)?
B: Do it yourself. I'm not your mother.
Close the door when you enter a room! Were you born in a barn?
The first implies that it's rude to ask other people to do things for you (unless they're your parent). The second implies that the person has not learned good manners, like closing something they opened, or putting away something they took out.
As for your example, we would probably tell the child not to waste water, perhaps something like:
This isn't your own house! Turn that off and stop wasting water.
Since I live in California, where we always have a water shortage, this sounds a little strange because it implies wasting water is OK when you're at home. We Californians tend to react when we see water being wasted anywhere, even in other people's homes.