I have 4 iPads set up with one ID. Mine, my husband's because I buy apps for him (we really don't share apps much). My ID is connected to a small stand alone bank account that we use for all online purchases. That way if anyone gets that info they will get very little money.
The other two iPads are the ones we gave to the grandkids when we upgraded. That way I can pay for their apps. Works out well. I get my grandson to steam my kitchen floor for a new app (he's 9) and my granddaughter dusts (she's 5). Sweet deal for me.
I have multiple iPads and iPhones on my account. I allow my 11-year-old daughter download privileges but I have my iPad set up to automatically download any apps purchased by another iDevice on the account. Gives me a way to monitor her downloads. She is great at finding paid apps that have gone free.