The Shoeless Joe Joe Junior Series: 1000JRIW Youth weighs approximately 355 g.
Generally, we suggest a mitt around 10" for a 6 year old player. This Shoeless Joe Joe Junior Series (1000JRIW Youth) has a 10" length and would be something for consideration for your player.
For a 9 year old that is playing a variety of positions (infield and outfield), I would probably recommend going with an 11" mitt over a 10" mitt like this Shoeless Joe Joe Junior Series (1000JRIW Youth). The 10" mitt would be a little small in my opinion. I might recommend a mitt like the 2017 Rawlings Gamer Youth Pro Taper 11" Baseball Glove (GYPT1-1B).