Here goes - I'll just put things down as I think of them, so it may be a bit haphazard!
How are you carrying the kids? Ours were both still small enough (ds just) to go in a trailer together, which was great because it didn't matter about the weather and dd could nap when she needed. Now that ds is bigger (nearly 5) he is on a tagalong and dd is still in the trailer.
Remember that while the best bit for you is the cycling, the best bit for the kids is the stops, so make sure you have plenty. We looked for interesting things on route, such as Orford Castle, or the Dinosaur Park at Lenwade just outside Norwich. Most villages have a recreation ground with swings etc, and we found these great for snack or lunch stops. Ask in the village shop if you can't see the playground.
We had more luggage than you, because we camped, but a bit of thought about packing helps. We had one small front pannier that was the 'office', with all the valuables such as camera, wallets etc in - when we stopped we only had to take one pannier with us. Another small pannier was the changing bag, with nappies, wipes etc - again you only needed to take one bag off to the changing area. Keep waterproofs easily get-at-able!
We cycled 30-40 miles a day, but this was actually a bit too long with all the weight we were carrying/pulling (I see from elsewhere that your dh is a triathlete, so presumably he'll be carrying the biggest load - unless you're one too?). Ds (nearly 5) is now too big for the trailer, and now goes on a tagalong - on holiday this year he did 23 miles one day, quite good for a littly we thought.
We took a couple of reading books for the kids, but didn't have room for toys. We took some felt tips and exercise books so that they could draw, and we collected leaflets and postcards etc each day and did a diary in one of the exercise books each day with ds - it helped keep him occupied when we stopped in the evening, and he still loves getting it out and looking at it.
That's all I can think of right now - I'll post anything else as it comes to me. We had a brilliant time - we're contemplating doing something similar again soon, maybe in Denmark. We might have to b&b it next time, though, because with ds on the tagalong we can't pull the luggage trailer, so can't carry all the stuff we need for camping.
Anything else you can think of, just ask. How are you getting to start and finish, by the way?