I need help creating a snapshot field in a unique parent-child relationship.

In my app, there is a relationship between two tables, call them Table A and B. In table A, there is a column called batch date. For this field, we would like to create a new field based off of it called initial batch date in the same table. This would essentially be the first value that was inputted in the batch date field (that field has end users inputting a value often). From the research I have done, the best way to preserve original values is using a snapshot field. From what I understand all I have to do is add a lookup field to the child table, then create a new field for a snapshot that effectively references this lookup field. The problem is in this relationship, table A is the parent and table B is the child. How would I create a snapshot field for table A in this situation? So far I added the lookup field in table B, then I created the snapshot there, and then added it as a summary field in table A to reference the original snapshot. This did not seem to give the desired result. Any suggestions? Open to anything. Sorry if it is not super clear as I am only a few months experienced in QB. Thanks,