I really have learned to trust IGN's opinion on game reviews. While I might not always like the titles they rate highly, I usually find their points valid which usually means I agree with the rating they give it within 10%. Going into their review section and looking at high scores can be a fairly good inidicator of what's hot and what's not. Often I wont read the entire review just the closing comments/summary.
The reader reviews on gamefaqs.com can be good too.
With that said, for PS2 the Jak series is a great kids game in that age range. Its youth oriented but has an all age appeal. Its adventure style with great animation, action, and dialogue/story. Its a good mix of different playstyles too with action adventure, problem solving, puzzles, and very good driving for a non driving game.
Tony Hawk series is also a very well made series as well mixing youth and older skate culture with pop culture within a very well made action/skateboarding game.
SSX is a really good snowboarding series.
As mentioned Rachet and Clank is also a good game similar styling to Jak.
There are alot of good sports games too like NBA street series, Madden, ESPN NFL/NHL series, etc. Alot of the best sports games have great playability even if you dont really care for the actual sport.
Of course tons of good racing games too from Gran Turismo which might be a bit advanced for a 8-9 yo to midnight club which has a easier learning curve to a degree and faster action, less statistics.
Big titles you may want to avoid is of course GTA series, resident evil, Metal Gear, Splinter cell, for its violence, and also it's adult themes. Popular RPG's like Final Fantasy are heavily story based which might be too much for a kid under 10. The WWE smackdown games are pretty well made and something commonly given to young kids but IMO the adult visuals in those games aren't appropriate for someone that young.
Anyways there are alot of option out there but it's up to you and/or parents to decide what is appropriate content while still being fun. Another factor might be playability and play style. Some games really require you to sit there for hours to get anything done while others make themselves easy to pick up for 30 minutes, play and then be able to leave it. This might be important for you to not promote game addiction. If you go to those game review sites and sort by high scores and read the summary and view some of the in game screen shots, you can quickly get a good idea on what would be good choices.
good luck.