Yup, get a tall 6 berth so you have a little extra space and standing room is more valuable than you think. An awning makes a lot of sense if its rainy – you can still cook outside the tent if you need to.
Self inflating mattresses are much warmer/comfier than airbeds and take plenty of warm clothing for the evenings.
Stick to campsites with a decent playground and make sure you can pitch near it within easy vision – makes a huge difference having the kids in eyeview without actually having to sit in the play area with them, you will find your weekend a lot more relaxing having a bit of precious "me" time.
We started going 2 years ago and went from a 3 to a 5 berth in 2 trips. Got the bug and i find it a great way of doing "not much" at the weekends – as a parent you will know how valuable that time can be. We eventually accumulated cooking gear, an awning, self inflating mattresses, little fridge/coolbox, warm sleeping bags, electric hook up, couple of folding chairs, couple of decent lights and torches and we're now sorted. Just add kids, bikes and kites.
This is ours