How to create Dropdown list of certain DocType's Child table's field(Custom script) in another DocType's field or Tables field

Parent DocType 1 : Item Process List
Child Table: Item Process List
Child Table’s Field: Process Name (one among of the many field)

Parent DocType 2: Process Log Data
Child Table: Machine Log
Child Table’s Field : Process group
Child Table’s Field Type: Table MultiSelect field

I need to create List for " Child Table’s Field : Process group " where user can select among the list.
The list should contains all the values of " Child Table’s Field: Process Name "
It will be very helpful if someone guide me to solve this issue.
Please share if you find any similar solved topic would be happy to learn from that and solve by myself.
Thank you.