erase ipad erases iphone?

When I bought my new iPhone 5s, I backed up my iphone 4s, then I completely erased it so that I could give it to another family member to use... however after I did this - my ipad was completely erased?

Why is this - is it because I use the same Apple ID for all my devices?

Is there a way to stop this from happening?

We're getting an additional family ipad - I want to erase one of them to give it to my youngest. I'm affraid if I do, it will erase all my kids and our ipads.

Please advise.

I have backed up all our devices today.

Thanks.