Just to add for anyone who is new to the topic:

When the accompanying parent travels on a redemption / award ticket, and they do not want to book a separate seat for the infant, the parent needs to buy a cash ticket for the infant that will be seated in their “lap”, i.e. travelling in the same seat as the adult. The price will be 10% of the cheapest available adult ticket in the same travelling class (I.E. economy, PE, business or first!) for the same sector. One cannot redeem such an in-lap infant ticket with award miles.

If you want your infant child to occupy a separate seat next to you, you can book this with miles. It will cost you the same as an adult award ticket.