I haven't been to Sealife in awhile but I went with little kiddos and we spent half a day there. I'd go to Sealife first otherwise they will rush through to get back to the park.
We could spend all day at
Legoland, I'd suggest skipping Sealife to get as much out of Legoland. Legoland is best for 3-8 year olds so there is not a lot of things to "skip." Devoting a whole day gives you time to explore and see all the details. Enjoy your trip