Hi,
I’m not sure I understand, but you can use vLookup both from the parent to the child and the other way around – as long as you have a field / value you can use to set up the query in the vLookup config.
Regarding populating values between parent and child: The default functionality lets you set values in the child item when you create it from the “Add new child” in your parent form. If you need to transfer values back to the parent when editing the child, you must use Custom JS. You will find some information in the forum.
One last note: the information about children that shows in your form only “lives” in your browser – drawn on the fly by the script. This means you cannot for example export list contents to Excel and include the vLookup child items.
Alexander