Most internal Oracle data structures that are protected by latches are protected by only one latch. However, in some cases more than one latch may be used. For example, there may be a number of library cache latches protecting different groups of objects in the library cache, and separate cache buffers chains latches are used to protect each of the database buffer cache hash chains.
Whenever a number of latches may be used to protect different parts of a structure, or different equivalent structures, these latches are called child latches. For each set of child latches of the same type there is one parent latch. In general, both the parent and child latches may be taken. In practice, however, the library cache parent latch is the only parent latch you are likely to see being taken, and even then this is a relatively rare occurrence by comparison with the activity against its child latches.
Somewhat confusingly, Oracle also refers to solitary latches that have no children as parent latches. So the V$LATCH_PARENT view contains one row for each of the solitary latches, as well as one row for each of the genuine parent latches. V$LATCH_CHILDREN has a row for each child latch. Thus, the union of these two views represents all latches.
The types of latches used by Oracle, and whether they are solitary latches or parent and child sets, varies with different releases of Oracle and operating system ports. The APT script latch_types.sql can be used to see what latch ...