My sim gave birth to triplets .. when she already had a toddler and an infant. When the triplets aged up to infant, since their mother can only do tummy time with one at a time, I hired a Nanny. But there's no way to tell the Nanny to do tummy time with them - she autonomously did that once with one kid, and then just stood around looking useless.
Infants can't do tummy time without adult help until they've achieved rolling over from their back to their tummy, so those first several tummy time experiences require an adult. This is the only reason I hired the Nanny. My sim was managing looking after everyone, but she can only tummy time one kid at a time and it does take a lot of time. I'm playing on normal life span, so they don't get a lot of spare time to try to collect all their milestones.
I expect the infants to be able to instruct the Nanny to do tummy time, or at least for the Nanny to initiate that repeatedly - but for toddler interactions with a Nanny the toddler initiates all their needs, so it would make more sense if infants could do that too.
