How do you stop a 18 month old from sucking there thumb! He is a daycare boy! He slobbers like no other when he is sucking it. It's gross and slobber gets all over. I'd like to stop it. His mom doesn't do anything to prevent it. I tell him to stop when he is doing it. Last about a minute and right back in. She said he does it only when he is tired. That's false as it's gotten worse than ever before. Help!!!