Try 7a Jesus lane. It's a pizza express but caters well for kids and coz it's an old gentle mans club you can eat in a lovely panelled library or an airy conservatory. Maybe pizza isn't your bag, but it's not a bad restaurant.
Carluccios- it has the space, caters well for all ages, is v friendly and of course the food, and is in town centre. Or ask about the private room in Browns on Trumpington St. The ambience there is great.