Is it true that Jesus said in Revelation 21:8 that all liars will go to hell? If so, what is the best way for a Christian parent to avoid lying about things like Santa, birthday presents, or surprise parties? Or if a suicidal friend makes you promise not to tell someone that they are about to kill themselves but you then need to tell that person, (already after you promised, because if you didn't promise, they wouldn't have told you and they would have died)?
The first question is puzzling since you can read the passage for yourself and see that it is true. So why are you asking?
The best way to avoid lying is to always tell the truth. "Therefore, putting away lying, "Let each one of you speak truth with his neighbor," for we are members of one another" (Ephesians 4:25).
Lying is not an option. Therefore, you tell your child the truth: the presents come from mom and dad. There isn't any Santa Claus, though some people like to pretend he is real. If you don't want to reveal a secret, like a present or a party, you just don't mention it. If you are asked, simply say, "You don't want me spoiling your surprise. You'll just have to wait."
I get told many people's secrets. I tell people upfront that I don't tell private matters to others unless it will result in harm. If they don't want to talk to me under those conditions, then it is out of my hands.