Needs to be big enough for 2 adults and 2 children...
I'm a bit uncertain about seperate sleeping areas as the kids are only 3 and 5, however I am not hugely keen on all being put in together as will then need to upgrade soon when they are big enough to be trusted not to run off!
A lot of the tunnel tents will have a curtain dividing the bedrooms instead of a fixed fabric 'wall'. You can usually tell on the diagrams as a curtain (that you can roll out of the way) will be a dotted line, and a fixed wall a straight line.
Hope that makes sense
We have a 3 and a 5 year old and all sleep in together. Makes reading stories at crap o clock much easier s you can all remain in situ, in sleeping bags.
Check out argos, easy camp, gelert and wynnster for tent ranges that are easier on the credit card.