Done it probably 7 or 8 times, last time being 2 years ago. If you are 6 foot or bigger, a single bed will be horrible, double maybe doable. That's just on a narrow boat though, the wide ones are better.
Also last trip someone caught a sickness bug and within 24 hours there were 5 of us with it. It's not very private and you definitely live on top of one another.
However, it's a brilliant, brilliant holiday and would thoroughly recommend. The best bits are finding a nice pub to stop near for the evening, and waking up in the morning to look out the window with the view moving along!
If you want an adventure check out Penichettes in France and all across Europe. They are bigger feeling boats (but shorter), and it's more interesting as some of the locks have lockkeepers, or are automated by a pole you grab as you drive along! Great fun. Disadvantage is the French don't 'do' pubs!
Prefer the french boats, but the English culture
In terms of kids, if they are outdoors interested then they'll love it. If they aren't, then it might be a bit dull. Consider bringing some other family, the boats can sleep up to 12 (8 in comfort IIRC) and whenever I went away it tended to be 3/4 men/boys on the bridge whilst the ladies stayed inside drinking/playing games. Haha. Minimises the cost too.