BA charge Infants 10% of the base fare across all travel classes. However, the taxes/fees/charges/surcharges element for Infants differs depending on the route. For example: Barbados 10%, Singapore 18%, Hong Kong 20% & USA 31%. Domestically it differs between airports.

I took advantage of last November's 50% redemption sale and booked my family First returns to Barbados. My 1 year old daugher was charged 7,500 BA miles + £28 for the taxes [10% of the adult charge less departure tax from BGI]. The redemption represented excellent value compared to the Infant cash fare of up to £719 + £28 taxes.