I'm going to lock this as I'm not even sure if it was still an issue after the infant patch.
If you do still have issues with toddlers, please create a new thread.
I see the same issue. Adding that this is not true for the siblings. They immediately know the toddler and can interact with them without any issues. I play with a single-mother family so I cannot say anything about the father / co-parent.
Just to add to this, this only happens with kids born after patch 1.34. In my game, I had households where the baby was just delivered. No skills, nothing at all. Even after patch 1.34, all the members of the family already knew the now born and could attend her needs (including her teenage sister).
I can also confirm seeing this is one save recently. There was a similar issue a long time ago that was fixed and at that time it was tied to the parent having the beloved trait (which is earned by completing the friend of the world aspiration). In my save the parents both have that trait, as does the other young adult living there and they all have to introduce themselves, however the sibling of the toddler does not and nor does another sim, who is a teen who does not have the beloved trait. Do any of your sims have the beloved reward trait?
Best regards,
Jennifer
While the matriarch of my 100 Baby Challenge does have the Beloved trait, I realized that the older siblings are also required to introduce themselves to the toddlers upon aging.
My sims just had a baby and did not have to introduce themselves!
@MapleSimmer Neither of them had the Beloved trait so I think you're probably correct about that trait being connected to the issue. One thing I noticed is that both parents had a lot of friendship with the baby right after she was born. In the saves where the parents had the Beloved trait, there was very little friendship with the baby. Hopefully the devs can do something with this information. Perhaps the old bug that they fixed was somehow reintroduced at some point.
I'm also having this issue I noticed once I got the beloved trait no matter what interactions with babies in the game they don't gain any relationship with sims who have it. I used the code to remove the trait and my mother instantly became best friends with triplets after one interaction. They haven't aged yet but I am assuming that solves the issue. I'll report back if it doesn't. xD
Now that you mention it, I didn't have this problem before completing the Friend of the World aspiration. I'm playing a 100 baby challenge and I didn't have a problem for the first 50 babies, which was around the time I completed that aspiration.
