When our oldest asks about Santa Claus, we just ask her what she thinks and never answer with a "yes" or "no". When she can reach the logical conclusion that there isn't one then she'll know there isn't one.
At least at our kids' current age (8, 5 & 5), other kids alone telling them won't convince them about it. We do leprechaun pranks, which we tell them are just us, which their classmates don't believe in (they've discussed this), and they still believe it's real.
They also think faeries are real despite us saying they aren't - their older cousin likes to play "faerie" pranks when she visits. They think we think they aren't real because, since we don't think faeries are real, they never interact with us.
At least at our kids' current age (8, 5 & 5), other kids alone telling them won't convince them about it. We do leprechaun pranks, which we tell them are just us, which their classmates don't believe in (they've discussed this), and they still believe it's real.
They also think faeries are real despite us saying they aren't - their older cousin likes to play "faerie" pranks when she visits. They think we think they aren't real because, since we don't think faeries are real, they never interact with us.