Ba is not under any obligation here - the passengers that were booked on a BA flight were rebooked onto another airline
You now want to book a new ticket on that airline for an infant
You are not entitled to a new BA award for the infant on El Al
If BA can issue a ticket for the infant on El AL, it would not be an award booking but there has to be a fare on which BA can sell - you may find that El Al is unable to sell the standalone infant, but if it can, it would be your best bet

Infant travel in conjunction with awards across carriers can be an issue

If you had had an infant booking at time of original rebooking, then BA would have , of cours,e transferred the infant