Number of Children:(as number of immediate sub-classes)
Greater the number of children, greater the reuse, since inheritance promotes reuse.
Greater the number of children, the greater the likelihood of improper abstraction of the parent class. If a class has a large number of children, it may be a case of misuse of sub-classing.
The number of children gives an idea of the potential influence a class has on the design. If a class has a large number of children, it may require more testing of the methods in that class.