Hi Jon,
I would second the advice about making it comfy for everyone. A decent tent with plenty of space will help a family holiday.
We use an Outwell tent (an older version of this one
http://www.outdoorworld.co.uk/outwell-hartford-l-tent-p-232.html (not recommending this supplier, just googled the tent!). We bought ours cheaply at the end of a season as it was... shock horror last years colours
My brother and his family use a Vango of similar dimensions. We have two kids who each get a room to themselves, which really appeals to them. SWMBO and I sleep in the central dome on folding aly-framed beds. We fold these up for the day, freeing up the central space. Whatever you get make sure it's waterproof. It's quite a nice feeling in your bag at night when the rain patters and you know that your safe and dry.
Other useful kit? Anything that you/they fancy! We are now finding that we are reducing our stuff as the children get older. When they were small it was handy to have a folding table (which hinges in half revealing four seats) as you can say "I'm cooking now so sit there and draw!" and they also found it easier for eating at. Now they are older they can sit around on the ground and eat. We only used it a couple of times this year and will not bother lugging it again.
We also invested in a portable loo which sits outside the inner tent in our porch area. Not very bushcrafty but campsites don't like you leaving a little pile of freshly dug earth with a note saying "I really wouldn't dig here!
".
If you have kids it is a huge boon not having to get up in the wee (pun intended) small hours when they need to go. When I was a kid these units seemed to be about the size of a fridge but ours is no more that 1'x1'x1' (or 30cm x etc). You can get biodegradable chemicals for them now too. If you are an adult they are handy too but erm... if you are a bloke you may find it easier to kneel rather than stand if you see what I mean. Also if you have a small boy with you (if you don't then you can skip this bit) then you may want to leave the porch groundsheet at home and just have grass below the loo. They are not known for their accuracy in the middle of the night
, I noticed that the plastic groundsheet became a bit wiffy after two weeks. Subsequent trips without it and the natural bacteria in the turf breakdown any 'spillage' without odour.
Above all have fun! You can always take them out in a bivvy bag with a spade later on when they are ready for it.
Cheers,
Steve