It's I saw you or I see you (if playing hide-and-seek or peek-a-boo).
When you say I have seen you, what you're really saying is I've seen you several times at [possibly] various places. Or that you've just seen them around somewhere or sometime. There is no indication of a beginning and end of the event; or if there is, you're unaware of the details.
Present perfect is used for events that began at an unspecified point in the past. You may or may not know whether it's still happening or not. It usually has relevance to the present,
As for I saw you, that's the simple past; and as you've suggested, it's correct.
The simple past is used for events that have only occurred once, were one-time events with a definite ending, and/or are not happening again (i.e I mean that specific event).
So, to return to the hide-and-seek reference, you use the simple past saw because you only saw them ocne. And once you saw them, that event was over.
Where it gets tricky is that I saw you technically means:
I saw you hiding in [the closet]. So you only say that after they come out of hiding, meaning You weren't very well hidden, because I
saw you the entire time.
If they're hiding, and you can see them, then it's obviously I can see you.
I've seen you hiding, means you've seen them hiding previously, possibly many times. It also is something that you would say if your child asked you to play hide and seek; in turn, you'd say:
Please. I don't want to play; there's no sense in playing since every
time we do, I've always seen you hiding. It's just too easy; I've beat
you several times already.
The best rule of thumb that I can think of would be that if it only occurred once, you have no reason to expect it will return, and you know exactly when it happened, then it's the preterite.
Otherwise, you'd use the present perfect.
