Word can't handle a one-to-many relationship in mail merge. You'll have to find a way to flatten that out.
However, there is a type of mail merge called a "catalog," where you can put data from multiple records on a single page. You'd use it, for example, to create a phone directory. You might be able to do what you want with a loop that pulls data for one parent into a cursor and uses that for a mail merge.
Tamar