Were these lap children or did they have their own seats? On US carriers children under 2 are allowed to fly on an adult's lap and no they don't get belted in (in the event of a possible crash landing they are even to be placed on the floor). This has been an ongoing debate as to whether it's safer to require children under 2 to have their own seats and be strapped in or for the situation to remain as is. One side argues that if parents are forced to pay for tickets for babies they will take longer driving trips and more children will die in auto accidents. The other side points to small children who've become projectiles and died in otherwise surviveable crashes.

In any event I've been on flights where the FAs have insisted the children be seated in their parents' laps during the situations you describe. I've also noticed foreign-based UA crews seem much more lax on following safety rules than the US-based crews.