Good sugestion by Mark, other possibles, (might be an idea to phone) are:
The Devonshire have a dining area that has french windows which open onto the beer garden, would probably take a booking of your size.
The Oddfellows have a room upstairs which you can book but I'm not sure for how many.
The Midland, well known for being children friendly.
If the weather is good the the garden at the Spring Gardens, with it's mini farm has got to be worth thinking about for the kids.
I think that about covers it.
Cheers!!