Mazel tov on being a wonderful Jewish mom :).
I think the language we use is important. On the one hand, you don't want to say "Santa is a lie." That puts other families down, and it gives kids a "truth" that they're just bursting to tell. On the other hand, you don't want to tell kids that Santa comes to other kids' house but not to theirs. Young children are so sensitive to exclusion and to the idea of being skipped over as it is, and Jewish children always experience some measure of holiday alienation: just picture the typical shop window, festooned with Christmas decorations, and with maybe one tiny menorah in the corner as an afterthought.
My plan (my son's only 2, so I haven't tried it yet), is to say "Santa is part of a different holiday. Some people have Santa and Christmas trees, some people have Kwanzaa, some people have Ramadaan, some people have Diwali, and we have Hanukkah, with the menorah, and dreidel, and presents for 8 nights, etc."
I don't know about explicitly asking a young child not to tell other kids the "truth" about Santa, though. That seems like an impossibly big responsibility, one that's sort of doomed to fail...
Best of luck. If you like, check in with me after the holidays. We can compare notes on what works!