There's no reason the child can't be "restrained" as a lap child during takeoff/landing and any turbulence, although I wonder if even just the regular lap belt would be better than mommy's arms. But then having the extra space to lay the child down would probably be appreciated by the parents. It's obviously less ideal from a safety perspective than a car seat, but probably better than a lap child.

OP should be aware that all of DL's business class seats are somewhat separated from one another. The "closest" are probably the middle pair on the 767s but there's still a fairly wide shelf between the seats.