I've been stuck on this problem for a couple days and was wondering if anyone had trouble with it before: I'm trying to make the NPCs in my new service pick up babies/toddlers on a lot and take them to the car, and adding a PickUpChild interaction instance to their queue doesn't get them to do it.

The service sim routes to the toddler and they get lined up facing each other, and an interaction with the service sim's thumbnail on it very briefly appears in the toddler's interaction queue (presumably it's "be picked up"), but it disappears so fast I've never even been able to check what it is. And then the service sim goes to the car and gets in, which is the next thing they're supposed to do except that they're not supposed to be alone, so it doesn't seem like trying to do this is breaking my whole service situation. So I'm confused?

Any thoughts on this would be extremely appreciated! I'm almost ready to just give up on handling any sims that can't walk realistically and have them teleport home while everyone else rides in the social services car.

Hmm sounds a bit like your interaction is missing a posture precondition.
You can look at FloTheorys Payforthis mod (give bottle interaction) for posture preconditions.

Ahhh you're right! This is just like when I hadn't set a posture precondition for a computer interaction! Thanks, I'll give this a try

Super late but I found what I believe was the reason this didn't work and wanted to put it in the archive in case it's relevant to others: babies and toddlers can't be interacted with by sims outside their household who don't have at least kBabyToddlerLikingThresholdToAllowInteractionsWithStrangers relationship points with some member of the baby/toddler's household (this is implemented by the method RunSpecialCaseAgeTests). So my fake social worker wasn't picking up the child because they weren't allowed. (It seems social workers and "childcare services"(?) service sims are exceptions to this, but my new service is not the actual social worker, so that didn't help them)