Hey Bryan - sounds like you are having a ball with your kid! That is the most important thing to keep in perspective throughout your time in karting - hope you guys have a long run!
Before even getting in to the karts out there Iāll repeat a few things that I say to all newer families in regard to working with young kids and how their money is best spent:
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The three most important keys to success - seat time, seat time, seat time. It sounds like you guys do this really well already!
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The single best investment is in coaching. Not overwhelming, immersive coaching that separates you from your driver. But coaching where the instructor understands that the hour you are paying for is as much about teaching you what to work on with your driver the next time as it is to teach the driver. And coaching is not changing axles, adjusting caster or moving seats - coaching should be one on two time with a kid & parent, not working on karts.
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Get a MyChron with a GPS, download race studio and learn how to use it. It is a very powerful tool.
A note about 1-3) Your driver will not be able to give you feedback and they wonāt have any idea how a kart feels different after you make a change. But never quit asking after each session and no matter what they say encourage the dialogue. In 4-5 years they may start giving you usable infoā¦ maybe. 
Also, āI donāt knowā is always a perfectly acceptable answer - they are really being honest.
As for kartsā¦ Usually the best move is to find a product that is supported locally or at the very least has a dealer/distributor with significant inventory within a dayās shipping time. You wlll bend, lose, break something eventually. Also someone that really Is vested in youth karting is a plus.
In terms of cadet karts that has been my primary focus for a long time. In particular, over the last five years, Iāve made the rounds through just about every major brand. There are some karts that just arenāt very good or tunable, some that you have to teach a kid to drive improperly to go fast, some that you have to tweak constantly but are wicked fast and then lastly some that are consistently good and easy to work with.
Unfortunately the kart you have falls in the first category. As the tires have gotten softer and horsepower has increased in cadets, the Arrow just struggles more and more. You may not see if now with a really young, small driver but there will come a day where youāll resort to pulling out a sawzall and hacking bars to try to free up the Arrow - been there, cut that. 
Many of the very biggest brands have built a kart primarily to run on the hard tire in Europe that also has a ton of built in rake with the differing tire sizes from front to rear. In order to make a Birel, OTK or some other karts go quickly you narrow the fronts, widen the rear, run spun wheels, etc and then teach the kid to slide the kart around the track essentially. Just not the platform to teach race craft and develop a driver.
Personally the CRG and Praga are two of my very favorite karts - the CRG is just wicked fast and the 900mm Praga in Rookie/Micro was just a beast. Both suffer a little though from lack of many people on the product right now with very limited dealer networks.
That brings us around to the current market which is a whole lot of Parolin based karts that work exceptionally well with the size and compound of tires we run in the US. While they are all similar, they all have some variations so no one is ājust like a ________ā in reality. Parolin, Energy, Merlin, Nitro, PDB, Benik and probably some others Iām forgetting are all really good karts. If you can find someone in your area that supports any of these brands you will be very happy. The best part about these karts is that they all come with (or should come with) a well developed baseline, a seat position for your driverās seat size and a pedal riser option that actually fits. They all also require very little tuning for a young driver in particular. You change a click of camber or front/rear width but for the most part you provide your young, developing driver with not just a stable platform but a consistent one. They just think about driving not the kart then!
Brief semi-shameless plug. We have run nearly all of them and chose the Energy as our brand. The fit and finish, and the complete as-delivered package, is head and shoulders above the rest. Just quality equipment throughout and Energyās owner Mick is crazy passionate about his karts, racers and brand which makes it super fun.