Thanks for all the tips. It sounds like I need to think about it a bit more.
A quick google had revealed Rugby, Stoke-on-trent or somewhere near Birmingham as possible starting places. A week itinerary from Rugby would only involve 2 locks whilst Stoke-on-trent could involve Alton Towers and some potteries and Birmingham could involve Cadbury's factory and sea world. I suppose I was hoping for some combination of that, but I guess what you are all saying is that it doesn't really exist!
We live near a canal and walk along it fairly regularly, so my 5 year old is the driving force behind the holiday as he loved the Broads and being on a boat that moves. I'm not that concerned about the 2 year old as I can always put her in my carrier to prevent her from disappearing in a lock. That's fine once or twice a day, but some of these routes have 10+ locks a day and that would really start to become a hassle.
I need to go and google all the recommended canals as I'm not that familiar with the names, but thank you all for the help and dose of reality.