This Condition located on the Family category tab in Search Builder can be added to a search to find which children have (or do not have) parents in the family. The Conditions are True or False.
Build your search to first find children using Conditions such as Position in Family = Child, and Is Member Of = Life Groups > Children. Then add this Condition. When it has a value of True, the results will find those with parents in the family; when set to False, it will find those without parents.
If you do not first limit the search to children, the results will find adults without parents, which is not what you want.
You can limit the search by using either the Position = Child or using an age range.
If you are trying to find out how many children enrolled in your Vacation Bible School do not have parents in your database, you could combine this Condition (using the Value Equal False) with an Enrollment Condition such as Is Member Of and select the Program and Division for VBS.