Update on our return flight: We successfully upgraded to business class, though the process was long and tedious via WhatsApp and phone. Hopefully, this helps other parents traveling with infants.
On our outbound flight from BRU, we were denied upgrades both via WhatsApp and at check-in. We were told that the only way to upgrade with an infant was by paying the full cash price for all passengers—using Qcredits or Avios wasn’t an option.

For the return flight, I decided to use WhatsApp again to upgrade my wife with Qcredits before they expired. The agent confirmed availability and, surprisingly, offered a second award seat. I assumed my lap-infant would block the upgrade, but she found a solution: detaching the infant ticket, upgrading both of us with Qcredits, upgrading the infant for just €200 in cash, and then reattaching the ticket.

It took a few hours, and I had to complete the infant payment via a phone call, but eventually, we received our updated travel documents. At MLE check-in, there was a minor issue with the infant ticket not being reattached, but a quick call to Doha resolved it.

In the end, we flew both legs in business class. So for parents with Qcredits and an infant—yes, it is possible to use them!