Dynamic data validation lists - from multiple select

Hi All

im hoping somebody can help, on getting me started on the below query

i have a relationship in excel which i want to use to dynamically filter a data validation list based on selection of the parent Collum, i have done this before but this time have added some VBA code which allows a user to select multiple options from the parent drop down




Please see attached example of data and desired output, for example if a user selects A & B from the parent drop down i want the next column to only filter the related items so in this case 1,2,3



Any help is greatly appreciated