Maz - It's jsut whatever gets you through the night. Don't they get it??!! If you do put them upstairs, chances are that during the night they will get up and won't be able to find you, or worse - fall down the stairs!
Gosh - you would love my mum - she is all for it. In fact when the kids stay at her house, she DELIBERATELY puts them in with her for the night!

My mum was belittled by her MIL for sleeping with us, not letting us CIO, bottle feeding us (she had severe mastitis and was told to suck it up)

That is why she would never question anyones parenting choices. She is now a very popular babysitter as everyone knows she will listen to the parents and respect their wishes.

Good luck