I assume you know that you can related the parent and child via c_numdoc field. do the following:
1) select parent
2) set relation to c_numdoc into child (child must be order to c_numdoc)
3) in your form the user would enter the desire doc number
4) with this doc do a seek on the parent
5) the records in the child table will be displayed with a grid.

i hope this helps.