Here's the American Psychological Association's take on it.

Some clinicians theorize that children understand and respond to trauma differently from adults.

Some furthermore believe that childhood trauma may lead to problems in memory storage and retrieval.

These clinicians believe that dissociation is a likely explanation for a memory that was forgotten and later recalled.

Dissociation means that a memory is not actually lost, but is for some time unavailable for retrieval. That is, it's in memory storage, but cannot for some period of time actually be recalled.

Some clinicians believe that severe forms of child sexual abuse are especially conducive to negative disturbances of memory such as dissociation or delayed memory.

Many clinicians who work with trauma victims believe that this dissociation is a person's way of sheltering himself or herself from the pain of the memory. Many researchers argue, however, that there is little or no empirical support for such a theory.
http://www.apa.org/topics/memories.html