It's all about how tall the kids are, especially the 6-year-old. You're right that a 2-year-old can ride many of the rides with a parent. You will miss a half-dozen of the big ones, which could really upset your 6-year-old.
The big issue is the 6-year-old's height. Most all of the rides, even the dinky ones, have a fairly high ride-alone height, like 48-54 inches. Check the website: If the 6-year-old isn't that tall, you won't be able to ride hardly anything. And almost all the rides pair you off in twos. They are sticklers for measuring you: my 3-year-old came in barely under the 40-inch height and they didn't let him on a 40-inch height ride.
I saw one family with three small children and two parents: the oldest child wasn't tall enough to ride by himself, so the father had to take them each one at a time on the ride -- and this was the little jeeps on a track, not even a rollercoaster.
If you have the option to do this on a day with others, even an older child that can ride with your 6-year-old, that would be better. Legoland is great that a 6-year-old can ride everything in the park, as long as they have a bigger person with them.
The zoo or Wild Animal Park might be better for a 6 and 2 without any other adults.
Please check out the website and figure out what you can really ride, before spending a lot of money!
PHXscuba