I've taken my kids to Disneyland a few times, the first time being around 4 for each of them. They loved it and remember it, but it was the time I took them around 7 or 8 that was the best overall.
Drawbacks: There's so much waiting in line and it can be REALLY tiresome for little ones (not to mention for adults as well), also the midday heat just makes everyone cranky ( I try to go in March or April, not during summer months).
It's expensive, but the absolute best time I've ever experienced Disneyland was during the holiday season. It's truly magical.