Populating Child Table Date Field to Parent Record

There was a step in my instruction to look up that summary field of the minimum record ID down to the child table. Once you do that, then when you make the next summary field, you will have a filter for it

Now you have not really explained to me if you have 10 children for the parent how do I know which child record's date field  do you want summarized. 

If you simply wanted to know the minimum date of all the children, then you can just do a summary minimum on that date field. But my solution is a little more complicated because I made the assumption that you wanted the date field from the very first child to be summarized, back up to the parent record.

Feel free to post back if it's still not clear or I'm not understand understanding your use case