Fetching single value from child table

How can i fetch a single value from child table of one doctype to another doctype.

I have a link field for " Shipping rule " doctype in Sales Order and i have created Custom field called " Shipping Cost".
My requirement is once i select the shipping rule the shipping cost should be fetched automatically. ( Below is the Sales Order Form)

But in " Shipping Rule " Doctype i don’t have “Shipping cost” or any other Amount field to use “add_fetch” to get it.
Instead we have " Shipping Rule Condition " Table which contains the field i required. ( Below is the "Shipping Rule " form fields)

Below is the “Shipping Rule Condition” Table. And the field i want fetch is “Shipping Amount”.

Hi you can try this way, I’m not sure but…

field: function(){
frappe.db.get_value(“Doctype”, { name: cur_frm.doc.name }, “field”, function(data){
cur_frm.set_value(field, value)