What adults think is having a blast and what little kids think is having a blast aren't always in sync.
Legoland targets younger kids. Mine, all under 7, love it and actually request going there more than MK. My 7-year-old son plays with Legos all the time. He is not really into anything at MK.
Parents generally have a much more emotional response to MK, something little kids don't always have. Our kids often ask to leave MK because they want to go back "on vacation" -- meaning the resort pool. Thankfully we have season passes and its not a big deal. We can do theme parks in smaller doses.
We were at Epcot recently where my kids love to go around coloring at the different countries. Another family came up, and their kids went over to color. "We didn't come to Disney to color," she barked at her kids, pushing them toward the door.
Anyway, I am rambling here, but your kids can have a blast at either MK or Legoland. If they are young and into Legos, they may actually prefer it.