We've just returned (just last night, can still hear "it's a small small world" in my head ).
I think the half-board vouchers are a good deal compared to the price charged when you're there. For example we had the premium vouchers which cost about 35 pounds per adult I think and we ate at the buffet at the Disneyland hotel where the advertised price is 50 Euro, which is around 45 pounds. The buffet was a good choice and the chracters came round each table and spent time with the children so everyone got their chance.
On arrival we were offered free vouchers for a character breakfast at Mickey's Cafe. I guess we were staying at a quiet time. It might be worth pre-booking just the meal vouchers and then actually booking the restaurants when you arrive to see if there are any additional special deals happening.
Some children are scared of the characters. ds now likes them (aged 5) but was scared when he went first, aged 2.5. (by way of explanation, yes it's something of a waste of time to take a 2.5 yr old and 6m baby to Disneyland but it was an extended family trip with older cousins!). I mean properly terrified, screaming, fast heart-rate. Obviously we kept him away from the characters after his first sighting but it might be something to keep in mind, for your younger one. dd (nearly 3) is completely different and loved them.
There's definitely plenty to keep your six year old happy. We were there with our two for 3 days, 2 nights and had no problem finding things to do, although I had to impose some limits (i.e. no more than once go per day on "It's a small world" for the sake of my sanity). Buzz Lightyear ride was a popular one for the 5 year old. Snow White ride is unexpectedly scary (they pick the darkest scenes), also Pinocchio. Peter Pan is good but quite dark inside which was not good with my two.
We had lovely weather this trip (previous visit was very wet!) and we could spend some time just letting the children play in the play areas, and explore the different parts.
Definitely bring fruit!
Coffee is uniformly revolting (they have done a deal with Nescafe and it's all you can get)
You should probably be aware too that there are ways to spend money everywhere you turn. Endless shops and food stalls. It may be sensible to set expectations re spending with the children before you go? I agreed with ds that he could have some Buzz Lightyear pants(!) and one toy and that was going to be the limit so there was no point in keep asking.
Have fun!