Yes there will be other families with children, the transatlantic crossing is a formal cruise with cunard and if you are happy to have that with your familiy you will have a fantastic time. The QM2 has a dedicated childrens club as well:
'Queen Mary 2 welcomes families to The Zone, a beautiful Nursery and children's centre full of toys, games, and activities. This area, located on deck 6, is supervised daily by fully trained nannies and youth staff. Older children will enjoy activities offered by the Youth Staff at The Playzone such as sports, disco, scavenger hunts, and arts and crafts. Queen Mary 2 also offers a family pool area so families can enjoy the outdoors together'
So you can also do your own thing onboard.