How Programmatically set child table coloum value?

Hy @nelson_d

Try this code

frappe.ui.form.on('Child Doctype', {
	a: function(frm,cdt,cdn) {
		table_ttl(frm,cdt,cdn);
		auto_cal(frm);
	},
	b: function(frm,cdt,cdn) {
		table_ttl(frm,cdt,cdn);
		auto_cal(frm);
	}
});
	function table_ttl(frm,cdt,cdn){
		var row = locals[cdt][cdn];
    	frappe.model.set_value(cdt,cdn,"c",row.b+row.a)
	}

I hope it’s working
Thank You!