Hi...Here's a quick update about the situation. I sent an email to Carole Kasper, Manager – Customer Care whose email address was listed on Eliot Reports UA Page. A few days later I received an email from someone on her team saying they had resolved the situation. See excerpt from email below.

After reviewing the details of your booking, I see that Ruby XXXXXX's reservation is separate from yours, and as an unaccompanied minor (UNMR) in a separate reservation, Ruby is unfortunately not eligible to sit in Economy Plus. However, since Ruby will be traveling with you, on a separate reservation, she is eligible to be seated with you in Economy Plus.

To resolve this, I have worked with our Reservation Team to make the necessary adjustments. I'm happy to inform you that Ruby has now been reassigned to seat 33B for both her outbound (EWR-CDG) and return (CDG-EWR) flights. This way, you, your wife, and Ruby will be seated together in the cabin, making for a more comfortable and convenient flight experience.

That's exactly what I expected the two premier desk agents to do, and I pleased with the result...But I'm still wondering why the agents couldn't do it and I had to escalate as I did,