We're planning our first Disneyland trip with our 16, 12 and 4 year old daughters. I highly recommend getting the free Disney vacation DVD (my husband requested it but I think you just need to go to the Disney website) because they offer up a lot of tips including Disney for younger children. Right now we're planning either 3 or 4 days . . . I'm leaning towards 4 but I want to have a couple of days available for other stuff like going to the beach (we've never seen the Pacific Ocean before!) and maybe to Legoland (which is about 1 1/2 hours away). I think 4 days will be good because I know my 4 year old will only be able to handle so much in a day. I learned this from experience because we went to Disneyworld when my 12 year old was 4, and after a few hours she was ready to go back to the hotel and hang at the pool.
We are staying at a good neighbor hotel -- the total package is half the price from staying at the least expensive of the Disney hotels so if you're budget conscience it's definitely the way to go. If you are a AAA member then they have a lot of good resources -- their travel services are the ones who helped me book our hotel and there are a couple of AAA perks thrown in as well.
Get your hotel booked ASAP -- they are already filling up fast for the summer. We had problems finding more than a couple of hotels to accomodate us because so many of them were already booked up.
We're very excited to go and I hope you have fun!