No, as you'll end up split across two PNRs
All the passengers on a booking have to be in the same fare bucket on the same sector.
As you have infants in the booking you'll be able to assign seats at the time of booking, but the last passenger will either have to pay to select a seat or just take a risk. BA is supposed to guarantee that the child always sits with the adult.
Have you looked at buying the extra Avios as well to hit your target, and/or paying cash for the infant tickets?