WIBTA to not tell the parents something vital that their baby did?

Hello! I (F 23) am a nanny and accidentally experienced something with the baby I watch (M 3mo) and I am torn whether to tell the parents or not. A little back story, baby boy is the youngest and LAST child that the parents are going to have. Mom had him by C-Section and, since she didn’t want to have any more kids, got her tubes completely removed during the procedure. He was a suprise baby, as she was planning on getting her tubes removed over a year ago, but got pregnant with him.

I stay home alone at their house with baby all day while the older kids are at school and parents are at work. During tummy time with him on his play mat… he rolled over for the first time. I have been having my phone close by the past week in case he did it so that I could send it to the parents if he happens to do it, but I left my phone on the couch and it happed so fast there was no way I could have grabbed it and took a video in time. I picked him up and got a little emotional. I was so happy to be there to experience one of his firsts… but I was also sad. Then… I became torn. I thought that maybe I shouldn’t tell them. These are the LAST FIRSTS that they will get to experience since they will not be having any more children. If I don’t tell them, then when he does it in front of them, THEY get to experience his firsts.

The only reason I’m torn is because every day mom comes home she always talks about how she thinks he will roll over anyway now, and I want so badly to tell her but I don’t want them to realize they missed out on something they will never get to experience again. So WIBTA?

Edit: Many are concerned that baby might be in jeopardy because of swaddling. A few weeks ago when parents saw signs of rolling over they cut all swaddling. So no he is not being swaddled