If you can stay at the Legoland Hotel, it'll be a really great experience. It's right there on site, so it's easy to go back to the room for an afternoon rest. You also get early admission (one hour) to the park each day. Only six rides are open early, but it's great to knock those rides out with no lines at all. You can get hopper tickets that allow you in and out of the waterpark and/or aquarium.
In the hotel, they give you a new Lego to keep each day. Last summer my kids mostly got mixels. They loved coming in the room and finding them each day. The lobby has evening entertainment, a big castle to play in and thousands of legos to build with.
The park isn't that big and we were able to go on all the rides we wanted each day that we were there. We would go for a few hours in the morning, spend a couple hours back at the hotel resting, then head back to the park in the afternoon for a couple more hours. some days we did the pool in the afternoon instead.
We stayed for four nights and it was a bit too much. I think three days would be good.