We (myself and wife) flew to Japan in First with our toddler. Outbound, she was 1, inbound she was 2. We paid 10pct of the Avios for her ticket. Outbound she was a lap child (but thankfully was a spare seat for her), and inbound she had her own seat. Whatever agent said when they tried to charge you, they were wrong. If legs are on each side of 2nd birthday, they fly for 10pct of Avios, and get their own seat on return

Just make sure you check the “ghost booking” that gets done for the infant, and will then be converted to a normal ticket with dedicated seat