Do you really need to store it or just display it?
A calculation field defined as: Max ( ChildTable::Field )
will display the maximum value of Field over the set of related ChildTable records. This is an unstored calculation. If, for some reason, you need a stored value, a script can use this expression with set field to set a number field in the parent record and this script can be performed by an OnObjectSave trigger on ChildTable::Field to update the number field every time a value is added. The same script would need to be run after a child record is deleted as well.