If you change the apple id on one device, is the id locked to that device for three months?

I bought two ipod touches for my children last week (all kids under 10 if it makes a difference). Child #3 already has one. When I bought them, I was told that I can download movies on each for an upcoming long distance trip. If child 1 wants to watch something downloaded on child 2's ipod, they can sign in under child 2's account and watch it on their own ipod BUT it won't be saved to that device.

However, I went in today to get them set up. I asked about this, and music and other things. I was told that if you sign in on one device under a different apple id, that device is locked to that apple id for three months. Its apparently an effort to stop pirating. I was told its the same for music. If I buy a song on my account that my 5 year old wants, I have to buy it twice because if my son signs onto his ipod under my apple id, the ipod is then under my account for the next 3 months, along with all my games, music etc. and his apps are no longer accessible to him.

Is this acccurate? Honestly, I"m afraid to try. I don't want my kids to be stuck with my apple id, which has no apps or antyhing attached to it other than some music.

BTW I had to set up their own itunes account despite that they're too young to have one because we're maxed at 5 devices (2 iphones, ipad and our own ipods). We couldn't add any others so I created a itunes account for my two youngest to share and a separate account for my 10 year old so that he could have his own facetime.

Can anyone help? Can I sign onto my kids ipod with my id without having them locked out of their apps? Can the two of them share a movie?