You could use a shortcut in this scenario.
If the attribute in the child BO should always reflect the value in the parent BO, a shortcut would be a more convenient solution.
If you cant use a shortcut for some reason, here is another answer to your question:
OverhaulSlots_ChildDB is not in the context.
Here is one way to do this:
If OverhaulSlots_ParentDB.PlanSlots WAS CHANGED Then
FIND OverhaulSlots_ChildDB WHERE (OverhaulSlots_ChildDB.OverhaulSlots_ParentDB=OverhaulSlots_ParentDB)
OverhaulSlots_ChildDB.PlanSlot=OverhaulSlots_ParentDB.PlanSlots