two ipod touches one apple id

Hi

My two kids both have an ipod touch, and I have the apple id on my computer. However when my daughter sends and receives messages from her ipod my son can read them and join in conversation (which is annoying my daughter) how can I stop this happening? Do i need to set up a separate apple id for both ipods or can I still be in control with one apple id and they have seperate passwords? Sorry I am not that techically minded and have searched the help section but no helpful! 😕