It is best to play by ear (Mickey Ear of course) ... If you travel and are pooped like we were it is just as exciting to get to know your hotel and pool. However if you arrive and feel ready to go you could get a lot done in a few hours at MK. But I would avoid planning on going to avoid expectations and just see how you feel when you arrive.
Ditto to what Jollean said, it's so hard to know what your going to feel once your off that plane, specially with little one's. I was thinking do you really want to waste a days ticket for a pretty busy park, chances by the time you get there you won't get fast passes for a time that would work for you, it may run into the time for your ADR's. Since your asking, I'll put my other .02 in

I would most certainly (if this was me, and I have a teenager and a 8 yr. old) check in and just take it easy for the night. I know how exciting it is to want to get to the MK, but I'm thinking that with little one's they won't want to leave once there.
Now,

if your really set on going then you could do something else concerning your ADR that won't have you leaving the park. You could always eat at Tony's Town Square, it's right inside the front of the park to the right, the kids would love this, or you could eat at The Plaze,(the end of Main St. on the right) it probably wouldn't be to hard to get in. If this is a big no because of the prized ADR that you already have, then I would check in, relax, maybe take the babies to the pool after you unpack. And yes I would unpack, at least for you and your dh, it makes things sooo much easier for getting ready.
Hope you have a magical time with your family no matter what you decide to do. Remember these are only our opinions based on our thinking and experiences, it doesn't mean diddle unless you want it to. LOL
