[AccessD] Increment Child #s

Assume:

T1 - parent file, say CustomerOrders
T2 - child file, say CustomerOrderItems

Goal:

Inherit the PK from T1, but add an Item# column that enumerates the Items on
said Order, so...

CO1
    Item 1
    Item 2
CO2
    Item 1
    Item 2

I wrote some code to do a Dmax() call using the Parent PK but yuk! I'm
trying to find a slicker way to do this, ideally so that I can default the
Item# value for the next new record. Any ideas how this might be done?

TIA
A.