3 ipods one account...how do I seperate them?

OK I have an ipod classic which I connect to my computer and update it as I need. My 2 children have ipod touch 4's. They are both using my emailaddress/itunes account. They do not/have not synced these with a computer. Using them wirelessly only (so far) but when they download apps from the app store they both get them. When they use the messaging apps out there they both get the texts from people sending them. How do I stop this and seperate the 2?