I would skip LegoLand and buy 3 day tickets for Disney.
My DD(now 10) & I went to LegoLand last May with a friend and her DD. My DD is a HUGE Lego fan! We do the free monthly builds our local mall. She has a ton of sets and likes to build her own things. We enjoyed LegoLand but it is no Disney. You should look into the ride restrictions there. You might have issues with 3 kids and 2 adults. The lines were much slower moving and the rides not as nice. The mini land was our favorite thing but it was so hot (in May) and certainly not worth giving up a day at Disney to see. We also enjoyed the gardens from when it was Cypress Gardens-the banyan tree was awesome. I'm not saying you won't have fun at Legoland but for the price I would say you could get more out of seeing more of Disney.
Your DS should be tall enough to ride everything at 6 unless he's very small. My kids were riding Test Track at 3.
We will have to give your 3 day ticket idea some thought.
My DS is very small for his age...he will be 6 in less than a week and is just over 3 ft! For the most part people think he is 3...maybe 4.