Those are definitely cute interactions! I didn't notice them til around generations, but I've only been playing the Sims for one or two years. Now here's something different I saw...
My sim child was at school and was invited to her friend, ole' Sam Sekemento's house. At his house, this is actually kinda funny, poor Yumi had three nooboos and a child on her hands. (Thanks to Twallans Story Progression mod, Tamara Donner and Leighton had gotten married and had three kids with another on the way.) In a baby crib, was an infant and with my child sim, I clicked the nooboo and it had "pick up" as an option, but it was greyed out because my sim wasn't very well acquainted with the family. Hmph. So, does this mean children can hold infants? Well, to continue my research, I had her go home and I clicked her own infant sister, but no interactions available. Strange...