I don't know which rolling over is the milestone for babies. Is it front to back or back to front?
Linus rolls over from front to back accidentally sometimes. I don't think that he has figured it out and it is something of a surprise to him, hence the outburst at the end. Mommy was there already but did immediately rush to comfort him.
No babies were harmed in the making of this film, although it is possible this may come up in a therapy session much later in life.