Hi Col, welcome aboard
We have a 12' with fully enclosed annex, & it does fit on a site in caravan parks, even with the guys ropes out...but just.
As smocky mentioned, the main bed is there on your trailer, so the rest of the main tent well more than accommadate beds for 2 + cot & clothes. Keep in mind, you're camping & not in a house, a lot of time will be spent out doors kicking a ball - chasing - swimming - exploring etc.
The annex is cooking, sitting & eating, & if the walls aren't up, it makes it seem even bigger
As I said, we have the 12', but it's only 2 of us most of the time, & it seems like a mansion. We have at times had 2 kids (10 &
plus another adult, & except for the kids clothes thrown everywhere, no one complained of claustaphobia(sp?)
I'd even go as far as saying a 10' would suit, but no smaller. As Hem said, they're be a bit easier to put up.
Seen any CT's in the flesh, that'd give you a better idea of size needed for your comfort.