My kids aged 6 and 8 still think Legoland (California) is the greatest place they’ve ever been, and they have been to both WDW and Disneyland and several amazing beaches, resorts, destinations, etc. They are somewhat wimps when it comes to big rides though. If your kids like thrills, they will be bored at Legoland. If they like legos and tame rides, they will be in heaven.
The water park in California was also great. I can’t speak for Florida, but I’m sure it is similar. They were afraid to go on most of the slides, but there was plenty of other fun stuff.
We managed to see most of the regular park and the water park in one day, though I was glad we had a second day to catch what we missed.
Disney’s water parks are better than Legoland’s but comparable for kids, unless they like the big slides - if so, Disney would be more fun.
I wouldn’t count on a Halloween party this year, but it’s possible.