We are local, so I know Legoland (and the zoo and Seaworld) really well. We've had memberships since the kids were small.
It may depend on what you want out of the day. If you are looking for rides and such, Legoland isn't going to provide that much for a small child (not that there are many better options in So CA). However, if you are looking for a few low key rides (with parents) and a place to play, then I think that LL is a great place for toddlers. Its less busy than Disney or similar. It has great play places -- musical fountains the kids can control, play village for free play, legos to build with, a great larger play structure, water playground with separate toddler-gentle water feature.
None of the rides that don't have height limits are long or scary. There are a couple of small roller coasters but they are 40 inch limits. Most of the gentle rides that I can think of are OK for all heights -- I think I can come up with about 10 that don't have limits on them.
Its definitely not more than a day's event, but can be a really fun day.