Maybe the manasikara cetasika is something with it. To activate sanna, the memory loading process is needed. But the baby has no experience to load. Therefore, there is no sakkaya, and sakkaya ditthi.
So the initial experience of lifetime is important. Because it has a lot of effect on the baby’s early impression of the world. Is this topic related to psychoanalysis? Because I’ve had a familiar ring about this content.