I would definitely ring them, they are really good at finding the best value deal.
Staying onsite also gives you perks such as the early magic hours which allows Disney hotel guests access to the Disneyland park at 8am, 2 hours before it opens to the public.
When you're staying in a Disney hotel, the day you arrive is the first day of your park tickets so your plan to stay overnight in Dover before is ideal. Get a really early ferry and then you've got approximately 3 hours to drive to Disney when you get to France. As for driving, it's so easy, it's all motorway and they're not busy at all, it's not like here. Even in the mornings when people are going to work, the roads are absolutely fine. Remember to consult the AA website though because you have to carry certain safety equipment in the car for France such as a high vis jacket for every passenger and a warning triangle etc.
I know you plan to go by ferry but if your budget will stretch, have you considered the euro tunnel? We've done both, we did the ferry first, but then we used the euro tunnel and I've never looked back! It is soooo easy and so quick - 25 minutes! It is more expensive, we paid £130 return, but it's worth it to me because it's so much quicker and simpler than the ferry. We are on a budget but I still turned down the offer of a free ferry from Disney - that's how much I love the euro tunnel! Hahaaa! I was sceptical the first time we used it because I didn't know what to expect but I wish we had done it from the first trip instead of spending nearly 3 hours on a swaying ferry. For us, it's the difference between arriving at Disney at 10am on our first day (we would be in the park by 11am) or arriving at 1pm (and getting into the park at 2pm). It also means we can stay much later on our last day - we leave around 7pm to catch a 11pm eurotunnel. So as well as being simpler and quicker, using the eurotunnel equates to at least another 6 hours in the parks which we wouldn't have if we were going on the ferry. Just something to think about because it's stuff like this that you don't realise until you've been.
With you having 3 kids (like us), Davy Crockett ranch will probably be your cheapest accommodation, it's about a 10 minute drive away, but it has a pool and the cabins give you more space than the hotel rooms and they have 2 bathrooms which is a plus! I don't know about your kids, but mine have a talent for all wanting to use the toilet at the same time so having 2 is great!
If you have the half board meal plan, you get a breakfast bag each morning which has stuff in like milk, cereal, baguettes, jam etc, basically a continental breakfast. A great tip someone gave me was to take your own breakfast stuff with you to France and then take your Davy Crockett breakfast bag into the parks with you and have that for lunch instead! You're not allowed to take massive picnics into the parks but they know it's your Disney breakfast bag and they will let you take it in. So if you bring cereals, fruit, brioches (or whatever you like for breakfast) with you from the uk, then the breakfast bag covers your lunch which saves you money and saves you making sandwiches every morning to take with you. Also, take snacks if your kids eat them, such as crisps and fruit etc. There is a supermarket but it always seems a waste of precious Disney time to me, I'd rather take it with me in the car and not spend my holiday time in a supermarket when I could be meeting Mickey!
Hope that all helps and that you get a good price from Disney. Come back and let us know what you do! x