here are my two cents...
legoland...your children will love! it's definitely geared towards that age group. is your 6yo a boy? if so, and if he's into bionicles, he'd love it there! my DSs were so into bionicles, especially our oldest, and when we went to legoland, he just loved it!!
calif science center...it's pretty cool. lots of hands on type activities & fun to explore, with that being said...lots of school take their field trips there, since it's free. could get a bit "chaotic" & may be overwhelming for your little ones
la brea tar pit...pretty interesting, but i don't think the kids might enjoy it as much. there's the natural history museum near by, and it's got a big dinosaur that they might like. and i think it's free the first (or second?) tuesday of every month.
someone mentioned kidspace in pasadena. our DSs loved it! they just remodeled it a year or two ago. and it's expanded a lot! they have a bike track where the kids can ride bikes/scooters, and they provide helmets as well. and depending on the time you go, they may have some special events happening that coincide w/the season/holiday, etc.
hope it all helps. have a great time visiting california &
disneyland!
