The specs actually aren't too bad on that stock servo if they are close to accurate. Might be worth buying just to not potentially have to screw with anything.
The listing for the Redcat servo doesn't list the spline count of the output but pretty much all servos come with horns. The only issue might be if the redcat needs a very specific length or offset on the servo horn and it doesn't have the more common 25T output that you can swap on to the aftermarket servos.
All that said, the cheap Amazon servos, and most stock servos aren't "good," but it probably isn't worth putting a good servo on the truck for a kid.
Thanks for the responses. I told him to stop doing the moving the wheels back and forth real fast thing. Mine is still working fine. Already breaking parts at 10. Now he has to work to pay for his own broken things.