Calculation of children in list of parents/children

Hello,

I have a Bill Of Material (BOM) list, where the main item (parent) has several subitems, which in term may or may not have subitems again.

It looks like this:

and so on.

So the item "A" is made of 2 subcomponents, 1 x "A1" and 5 x "A2". 

In its turn, "A2" is composed of 2 x "A2a" and 1 x "A2b".

Or otherwise represented:

• A
  • 1 x A1
  • 5 x A2
    • 2 x A2a
    • 1 x A2b

So A is finally composed of 1 x A1, 10 x A2a and 5 x A2b. It's this final level I'm interested in.

Same for B, etc.

I have also a second table, which is a list of quantities of the main items (parents that are not a child). 

What I need, is a calculation of the quantities of the "last level children" (so children that have no children themselves).

So in this case I would have:

etc.

I am familiar with Power Queries (using mainly the UI) and formulas. No VBA please (if possible).

How would you go about this? (Taking into account that subcomponents may appear in different main items, so A2b can appear in C, F, G2, I3c, etc., each in different quantities).

Many thanks in advance!