I ended up calling the Elite line in the Netherlands to help out with this, and not only were they able to fix the problem, they told me why it was not working before, and when the call got disconnected while I was on hold the agent phoned me back on my cell phone.

I was trying to seat the child on the window side, and they told me that the oxygen masks suitable for children on the 789 in WBC are in the aisle seats only. So let this stay here for future reference in case anyone else travels with a child in 789 WBC!

Very happy with the service I got on this call, I will probably just dial NL for future assistance. In North America there seems to be no Elite line as far as I can tell.