Arrange for a central meeting place in case anyone gets lost or separated. Disneyland has a very good lost kid retrieval system (and you should note the pick-up spot near the first aid area at the front of the park), but ideally, you don't want it to get that far. Identify a spot that everyone can remember and have everyone go to it the minute they realize that someone is missing. There's a statue of Walt Disney and Mickey Mouse directly in front of Sleeping Beauty's Castle that makes a terrific meet-up spot.
Single out the rides and attractions that are appropriate for your kids and go to them first. Roller coasters have height requirements and certain other rides such as the Haunted Mansion may be too intense for the very young. If you have young children, center your attention on Fantasyland and Mickey's Toontown; they have the most rides catering to the very young and they fill up quickly.
Take steps to avoid long lines. Nobody likes waiting, especially with kids, but there are some strategies to help you duck the extended waits. Arrive early and go on the rides you most want to visit first (they fill up as the day goes on). Try to visit in the off-season if you can--sometime other than Christmas or the summer months--and think about going during a weekday rather than the weekends. Disney also has a "Fast Pass" system, which lets you get a ticket for a given ride at a certain specific time. Once you get the ticket, simply do other things until the appointed time, then show up for the line without the wait.
Get a stroller for the toddlers. Disneyland involves a lot of walking and young children will be much less cranky in a stroller. You can bring your own or rent strollers at the park.
Hydrate. Southern California gets very warm, especially in the summertime and even just standing in line can take a toll. Make sure all of the kids get plenty of water and have them wear sunscreen and a protective hat if the sun gets to be too much.
Take advantage of Disney's end-of-day souvenir collection plan. Instead of buying a souvenir for the kid in the middle of the day, only to have him give tired of carrying it around or dropping it on a ride, have it sent to the front gate to wait for you when you leave. Every souvenir shop in Disneyland can do this for you and it saves a lot of hassle. Alternately, make purchasing a souvenir the very last thing you do before you leave.
