FWIW it was so much hassle to add an infant to an IB+ award booking last year that I cancelled and rebooked the same IB flight via ba.com. You pay higher surcharges, but the infant ticket is done online at the same time for only 10% of the Avios required.

Add the infant sooner rather than later, in case you also find that you need to cancel the whole itinerary and rethink...