There is no magic bullet. Infants in conjunction with award tickets can (a) be hard to ticket and (b) be expensive

If you are travellling one way as the adult, you can only buy a one way ticket for the infant. The infant booking has to be tied to an adult booking

You could book an award ticket for the infant and the infant would then also have a seat.

If you want to be able to look at gettign a r/t ticket for the infant and reducing the cost, then you will need to adjust your travel plans