I've owned 2 Spiderboxes and both had splined axle & hubs BOTH sides. Your axle pic. is a mystery!! I can't imagine them having just one wheel pulling all that weight. It's your choice for an up-graded axle, there are many posts about axles that work & manufacturers.
I will provide personal experience about the heavy-duty BD replacement axle--ours lasted a month
But we drive HARD
Your kid's driving style may help you make a choice as to how strong it will need to be- also the terrain they're driving on makes a difference too.You have an opportunity to widen it for better stability. Any axle 1" or larger should work, I recommend 1-1/4" or 1-3/8" depending on your budget. You will need to buy everything having to do with the axle at this point to make it last if you buy the BD axle.
- bearings,sprocket holder,sprocket with matching bolt pattern to the holder,brake hub with rotor to match bolt pattern(rotor needs to be the same diameter as the original for the caliper to work),hubs to match the wheels you're planning on using.
I have a 1-1/4" hex axle that they don't make anymore from the Jehm "Green Machine", but it's lasted 12yrs. of harsh treatment. FYI-
https://www.buggymasters.com/forum/s...ead.php?t=1159
has lot's of info about ALL the mod's I've done to the one we still own & drive daily. There are many members who have "Beefed up" the rear axle- CKAU has a beautiful pictorial
https://www.buggymasters.com/forum/s...ead.php?t=2156