Hixster Bankside is good, very relaxed, not too expensive, they serve brunch all day and children under 10 eat free from 12-6. Quite a decent kids menu and art activity pack. We've been a few times and enjoyed it.
Thumbs up for Hixter. You're indoors with no view other than the art (which I like, and I like the interior in general), but service is great, especially with kids, and the food is excellent, as long as you like chicken or steak (and I love their steak and their roast beef).
Camino at the bluefin is also great with kids and has good food and service if you fancy a tapas brunch.