Some more datapoints:

1) Booked UA one-way flight for my 2 yr old son. Booked him as an adult. Total cost was ~$1,200 in TB credit.
2) The ticket became cancellable at 26 hours from booking for FFC.
3) Called into UA to book him on ANA metal utilizing the FFC generated for him. The year of birth actually didn't match up. Error on my part but it didn't seem to be a problem.
4) UA agent was able to book the ticket and linked his ticket to mines even though my ticket was booked directly with ANA (so PNR and ticket # was not in UA's system).

On the phone calling ANA now to verify everything good on their end before I breathe a sigh of relief.

To be honest, I didn't know what I would do with the TB credit. I typically don't fly UA metal and often don't pay for tickets at all. This was a rare case where I needed to buy a ticket and I'm glad I could make use of the TB credit.