This is an entry in our Disneyland 101 Glossary: Disneyland Resort definitions in under 101 words, from Mouse Brief.
Children two years of age and younger receive complimentary admission to the theme parks of Disneyland Resort and are referred to as “guests of Mickey.” These guests, under three years of age may enter either Disneyland Park or Disney California Adventure Park for free without admission tickets or park reservations. Of course, they do need to be accompanied by an adult.
If you are visiting with little ones, you may want to check-out our tips for visiting with young children, including the use of rider switch. We also have some more detailed advice about using strollers at Disneyland Resort.