Run Web Form Child Table Event?

like we add child in child table then i want to run some code?
Normal field in parent we use below code,

frappe.web_form.on('amount', (field, value) => {
    if (value < 1000) {
        frappe.msgprint('Value must be more than 1000');
        field.set_value(0);
    }
});

But in case of child table in web form how to run the above functionality ?