I dunno, I flew UK-IRL this summer and in both directions I just threaded the baby-belt (standard issue here, the cabin crew hand them out before the safety briefing) around the carrier and the sleeping baby.

In a full on crash situation infants are not going to do well, so optimising for much more likely turbulence injuries (for which having the baby strapped as securely as possible is definitely a good idea!) seems more important.