How to enforce that one row in a parent table MUST be associated with at least a row in a child table
There is no declarative referential integrity to enforce one or more children in SQL Server.  You have to enforce this with application logic.

Note too that if you are using a foreign key constraint on the child, you can't save the child until the primary key of the parent is known.  This means that you need to have a transactional wrapper (BEGIN TRAN / COMMIT) around the insert of the parent and it's first child, so that the parent is not kept unless the child is also successfully inserted.