I have never seen a needle guard on a machine - but that is an interesting idea. I think it would depend on if the 9 year old would be watched closely.
I have let my great nieces sew on my machine and I did not use a guard. They were much younger than 9
probably 4 and 6... but then I was right there with them. Also at that age they did not really want to sew for long periods of time, so that made it easier to watch them
Also some kids may not pay as much attention as others... here I am an old lady and I wanted a needle guard when I started to seriously sew several years ago (I was much much much older than 9)
as far as machines go... I am partial to Janomes. I love my Magnolia 7330 and use that to take with me when I go to classes (this was the machine that I took to let my little nieces sew on)