Paul,

In your description you don't give us enough information to answer the question. A good relational design closely matches the actual entities and relationships that exist in the business being modeled. To determine if a particular design approach is going to be effective one must know details about the business and the nature of the data being recorded.

The third normal form can help you in deciding if the FK's belong in Main or not.

3rd normal form says that the non-key fields are ONLY dependent on the Primary Key. That is that if the value of any other non-key field changes it will not affect the value of other non-key fields.

In your example of Main having FK's to both ChildA and ChildB, if there is any dependency on the FK of ChildX on the other FK for the other ChildX then it violates 3rd normal form and the dependent FK should be in the ChildX table where it becomes dependent on the primary key of that table.

It is perfectly possible to have a table that has two independent relationships to two other tables. In this case having both FKs in the parent table is perfectly fine and in fact is the normalized structure.