We must tell our children about Santa Claus (or Father Christmas)

We should not lie to our children!

Begging The Question

Assertion without any logical support.

We must tell our children about Santa Claus (or Father Christmas)

It is a tradition, not some sort of religion or any kind of destructive sect. Anyway, the child will know the truth some time.

Fallacy Of Is To Ought

The actual fallacy is "Appeal to Tradition", but this will do as well.