I got the tickets through JTB (japanican.com). It was a little cheaper than the advance tickets directly through Legoland Discovery. They were 1400 yen/person (adults and children are the same price, only age 2 and under are free). We had to pick a date and it was nonrefundable and not changeable, so I decided to play it by ear and buy it once we were in Japan. The fine print says that the confirmation needed to be printed out, so I went to the front desk of the hotel to ask if they could do that for me. The first person I talked to, his English wasn’t so great and he thought I wanted to make photocopies and showed me a price sheet. When it became clear we weren’t communicating, he told me to wait for the same nice lady who had checked us in (wish I had caught her name). She told me I could e-mail it to the hotel and come pick it up later. When I went to pick it up, she immediately saw me and came out from behind the counter with the print out, no charge. 
The kids have been to Legoland California (the full size theme park) several times since we go to Southern California at least once a year to visit their grandma. They LOVE it. Our last trip, we spent 3 days there and by the end DH and I were a little sick of it, but the kids STILL asked to go back as we were leaving the last day. 
So – I thought, well, there are some different things, we’re going to Odaiba anyways…I guess we’ll go. I would regret that decision – just tried to do too much! But more on that later….
There is an area with Lego models of a lot of the sights around Tokyo. Here’s Decks (the building we’re in), with the Fuji TV Building behind it.
Godzilla pops out and roars. It’s so cute and I love his shadow.
Tokyo Skytree…
…and the golden turd. 
There is a dark ride called Kingdom Quest where you shoot at targets, some of the targets are physical things (like a treasure chest), but a lot of it is on screens. It was a Tuesday and not that crowded, we went on twice with no wait. The other ride is Merlin’s Apprentice Ride. The seats are attached to hydraulic arms that go in a circle and you have to pedal to go higher. The first time we went, there was a line and we had to wait for the next, next cycle. But then we went again later in the afternoon with no line, we even stayed on a second time. There were 2 different 4D movies. We only saw the Lego Movie one since we’d seen the Chima one before. DS spent a little time building a car to send down a ramp while DD played with the big foam Legos. Near the exit is a big train table with Lego buildings and a Duplo Shinkansen to push around.
There is also a play structure:
And a snack bar, where I got a cup of very expensive coffee (a small cup for 400 yen!). The cup was cute though with Lego mini figure pictures on it….
Travels with 2 Kids 