Can anyone tell me what is best the solution to validate data in one one-to-many-form?

The relationship is defined in the environment.

Parent Table: Expense (docnumber, ... etc)
Child Table: Item (docnumber, duedate, value, accountnumber)

To each item I have to check if the due date is valid, the value is not zero and to update another table (month balance). Month balance table contains one record to each account/year/month.

My problem is to choose the best method to do the data validation in the item table (contained in one grid) and the update in the month balance table.

Does anyone have one form class to forms one-to-many or one sample with data validation in grids?