Lie, someday one schoolmate is going to tell them how is santa and its fine. Believe me, when they found out, they are not going to tell you anything. They will be kept it a secret so they continue getting what they asked for. I keep the secret for about 3 years after I knew. After my parents find out that I knew, the crappy gift era began.
I don’t think that you should be losing sleep with that one.
Take care,
Ismael