Ideally, nobody wants to lose a child at the Disneyland Resort. :-) If your little tyke goes missing (or you go missing to them), Lost Families is located at the end of Main Street in Disneyland and at the Baby Center behind Ghirardelli in Disney California Adventure. Ideally, Guests should check here first if they have not already chosen another location to meet with their kids.
If the child is under 12, Cast Members are instructed to take the child directly to Lost Guests and make the child wait until a parent or group member arrives. In DCA, younger children are generally held in the Baby Center behind Ghirardelli. Make sure that you inform your child to go immediately to a Cast Member when they can not find you. If you have lost an older member, over the age of 13, they will be allowed to sign a log at City Hall (or Chamber of Commerce in DCA) and leave. When you are unable to find a member of your party and you have checked Lost Guests, the Cast Member at Lost Guests will contact security and the entire park will be alerted about your lost family member.
You know where to find your lost group members but how do you avoid losing a member of your party? There are many things that you can do to minimize the chances of loosing group members. When you first get to the park, decide on a place that every member of the family will go to when someone is lost. Many guides tell you to choose Sleeping Beauties Castle as a meeting place. This is a good idea but… The Castle tends to be very crowded, and that makes it very hard to find people waiting for you. Also the Castle is a very large area, if you are going to meet there, decide on an exact location not just "the Castle." You should choose an area in the park that is not very crowded and that is a smaller area. The Hub, directly in front of the Castle is a good place. It's in the middle of the park so everyone in your group can see exactly where it is when you walk in. You should choose one of the eight character statues to meet at. This pinpoints your meeting location to one exact spot, that is in a central location and easy to find. There are also many other areas throughout the park that you might want to choose, so keep you eyes open and be sure to notify everyone in you group exactly where you are going to meet. And I mean exactly, don't just say, "Hey, if we get separated we'll meet at that Matterhorn thing." That gives a very large area where you could be waiting. For exact I mean something like "We'll meet at the Goofy statue in the Hub."
Another idea for helping you keep your group together is dressing everyone in distinctive colors, like bright primary colors, neon colors or a group t-shirt. If the older members of the group won't do this just make sure you do have something special for the younger group members, since they tend to get lost a little bit easier. Distinctive coloring allows you to find someone who has wandered off. Also when you are in a crowd make sure you take children by the hand, and be very careful to keep the group together. Don't worry about rushing through the crowd just make sure you can keep everyone together. This is also a good time to remind everyone, "Hey, if we get separated we'll meet (fill in place here)."
Many people also loose personal belongings in the park. If you are one of the unfortunate people you should first check at the location you think you might have dropped your article. If Cast Members at a given attraction or area find items they are instructed to place them in specific areas at that attraction or show. These items are not taken immediately to the lost and found on Main Street, which is located next to the lockers, half way up the street on the east side. Try to remember where you last had that item and where you might have left it. Once you have checked at each of the attractions or shows where you feel you might have lost the item then go down to the Lost and Found on Main Street. If they do not have your item you can fill out an identification card that will allow Disneyland to ship that item to you if it is found. There is a pretty good recovery rate for lost articles and you may just luck out. Also, if you happen to loose an item such as a hat or such on one of the faster attractions, just notify a Cast Member and they will be on the lookout for that item on the ride. They probably won't shut the ride down just for your item but they will give you instructions on when to return or how to get the item back.