Thank you, this worked for me.
Just note that the client script should be filled out on the actual web form, not as a custom script.
See the below example on the Job Application Web Form for the child table language.
frappe.web_form.validate = () => {
let data = frappe.web_form.get_values();
frappe.web_form.doc['language'] = data.language;
return true;
};
