Setting up multiple phones on one account

I currently have an iPad3 and 2 3GS all on my account so my husband and I can share apps.

We will be passing our 3GS phones on to our 8/9 year olds to use as iPods when we get our 5s tomorrow. I want to keep their apps on there and also add them on the new phones.

Is there a limit on the number of phones sharing one apple id?

Is this going to be a cloud mess? I am the only cloud user right now, and I just use it for iMessages and contacts between iPad and iPhone. If I disable that they will be fine, right? I don't want them reading my texts!

Am I missing anything important?