Thanks for all the suggestions. We tried Milamodo today and the boys enjoyed it. It was quiet at lunch so they didn't disturb anyone, but we could have gone on the patio if it was crowded inside. Good choice for family friendly. They also really enjoyed the small fountain at the front of the restaurant. Kept them amused while we waited for the food to arrive.
Next time we're in Halifax we'll try one of the other suggestions!
Sushi Nami on Dresden Row is good if you're with a few people because they have lots of space in the back of the restaurant for high chairs, etc. The food is reasonably priced, especially the lunch menu, and they have tsunami hours which offer a 20% discount on the regular price if you come in during those times (I think its between 3 and 5pm, and after 9pm but I'd double check on that).
Highly recommend their Fusion rolls. The Spicy Shrimp is delicious.
We love Doraku as well, great prices, always fun and busy. We also like Milamodo in Clayton Park. There are private rooms at both, and I believe both take reservations.
As an aside, we had a terrible experience at Sushi Nami Royale on Dresden, but that's not somewhere you'd want to take kids anyway.
I like Sushi Shige on Spring Garden Rd. Always the best fish in town. The only draw back is that I don't think they take reservations. They do have tables at the back that would accommodate a larger group.
Doraku (on Dresden Row) is also very good and they have separate seating areas as well.
Dharma sushi is probably the most affordable and maybe the best for a family and is pretty good.