How about turning to the child and quoting the scripture:
1 Peter 3:15
If, out of respect to the parent, you still want to plant a seed. The parent might catch on to it as well.
When the parent says: "Well I'm ready to make a defense", then invite him/her back without the youngin', and give 'em hell, if that's what you need to do.
Firm believer that it is non of our business to try and undermind a parent.