I am learning English and trying to teach my son to speak English in daily life. We live in China. My son likes to play with tap water and observe the water running out. I always wanted to tell him “Turn the flow down a little.” Would this language be natural or comprehensible to a native English speaker?
I would agree that turn the flow down a little is perfectly comprehensible, however, while I would not go so far as to say it is unnatural, it is not quite idiomatic either. It has a certain precision to it that one might expect from a scientist or engineer or just one who likes to speak precisely. (Ironically, the use of the word flow also has shades of meaning one might associate with certain alternative or counter culture lifestyles - hippies, surfers, new ageists, artists, etc).
Typically in this kind of informal context I would expect someone to say simply turn down the water a little or even, if one were right there and it is clear what one is referring to, turn that down a little. In both cases you could replace water/that/it with faucet/ spigot/ tap. As is true with many idioms - the grammar of this is somewhat wanting; water as a thing in itself is not something you turn down. You may turn the faucet handle or spigot (at least this is the traditional way of adjusting the faucet, though modern faucets may require more of a shifting or lever action then a twisting action) but this is not really what you are after - you just want the flow of water to be reduced!
It is interesting that your expression eliminates but implies the water while the idiom eliminates but implies the flow.