Sloane We've been taking 8yo DS who has ASD since he was 3. He loves lego, and his first stated job ambition was to work at the Legoland Hotel.
We started going before he was diagnosed, and have always just bought annual passes, so we've just continued with that, so afraid that I can't help on the best way to buy for a one-off.
However, there is an excellent ride priority access scheme - you need to take evidence of your child's need - the original consultant diagnosis letter has always sufficed for our DS. Then the disabled person gets a hand stamp and a card with 10 tear off bits, and I think up to 3 people who will be going on rides with them also get a hand stamp. That means you can have priority access to 10 rides.
Full info is here. We've not had to register in advance before, just gone to guest services either at The Beginning or in Heartlake Village - but that might be new for this year.
The staff have always been very helpful, including coming out to do our hand stamps when DS refused to go into the building.
Definitely take the buggy if DS gets tired, as there is a lot of walking. You can hire legoland buggies (but for toddler sized children), and there are loads left by the entrance to rides, so yours will not be the only one.
The ride guide includes helpful info on what the rides involve, so that you can pick ones that your DS is more likely to enjoy.
We have found that it helps DS to know at the start of the day what he is allowed to spend in the numerous lego shops, and indeed we often start in the Big Shop by the entrance, so that he can have a browse of his options so that he can be mulling on it during the day before making the final purchase. But that might just be our DS!
Hope you enjoy. We have had some lovely (and some challenging) times at Legoland, but with the ride access pass, it feels like we can relax and enjoy things more than most places. We often only go on about 3 rides, as DS prefers the play areas.