I have to wonder if the girls initially fell asleep in their car seats, and the men were "relieved" that they were quiet, finally, and they could do as they pleased while they slept.
Toddlers at 16 months old are usually very active, in perpetual motion, exploring everything, and very needing of constant supervision by an adult. At 16 months, they likely could walk, and were toddling all over the place. It takes an immense amount of supervision to watch TWO active toddlers.
How did the twins end up in the car all alone in the first place? Did the men take them out to eat, for example, and then they fell asleep strapped in their car seats, tired because they didn't have their usual naps?
I can't shake the sense that these men were somehow glad, or content, or relieved, that the busy toddlers (with mom far away at another crisis) had finally fallen asleep, and were "quiet", so they were free to binge drink between themselves and socialize. Then the men fell asleep, or passed out, and the fate of the girls was sealed. (JMO-- speculation on my part.)