I'm surprised people are so worried about this. You can get hurt doing anything, walking down the sidewalk. Mowing the lawn does not seem extremely dangerous. I can't see how he could get hurt by the blades in this circumstances.
But, then I'm not a worrier.
My husband works in another state 4-5 days a week, every week. I mow when he's home on the weekend and can watch the kids. Yes, it sucks to give up part of our precious family time, but I'm not willing to risk my kids' hearing and safety. They stay inside while I mow. We have a 1.5 acre yard and I'm on a riding mower so its really unsafe. When we lived in the suburbs and had a push mower and a small yard, they still stayed inside. I wear the big muffling ear covers to protect my hearing, I certainly wouldn't expose my children to it.
This article reports that 16,000 children a year are harmed by lawn mower accidents. I'm sure that every single parent felt the same way you do, that there was no way their child could possibly get hurt. A child lost their foot in our community riding on a lawn mower with her grandfather a couple years ago. I know that grandfather must have felt confident the child was safe - he'd been mowing for over 50 years, probably.