Go right ahead! Keep in mind I am guessing with this, seeing as Diagon Alley hasn't opened yet, but it is based on the assuumption that patterns will be similar to the first year or so of WWoHP craziness. These plans also assume that the Hogwart's Express will be running during early entry. If it is not, we will line up to ride it as soon the parks officially open.
You will want to make your dining reservations now. Use opentable.com . There are dining threads around, but I highly recommend Mythos (make sure you have the rice krispie sushi on the kids menu for dessert) and we also enjoy Confisco Grille and The Kitchen.
The important thing to remember is that while Diagon Alley (and to some extent, Hogsmeade, since it is on the other end of the Hogwart's Express) will be packed, the other areas of the park should be much more lightly attended, so you should still be able to move through these areas quickly using Express Pass. Being at a close hotel, you can take that afternoon swim/nap during the hottest, busiest part of the day. The Harry Potter areas will be least crowded during early entry and the last couple of hours at night, right before park closing. Try to take advantage of all of this and you should be okay.