Below is a diagram of positive and negative caster that I found online.
Negative caster causes a vehicle to become unstable at higher speeds while positive caster causes a vehicle to be more stable at higher speeds. But having too much positive caster can cause the Power Wheels car difficult to steer at higher speeds. The current caster on the Power Wheels has a caster of -7 degrees. I measured it by using a level as my vertical axis and measuring the angle of the spindle bracket from the level. From doing some online searching, I found that a good amount of caster is +10 to +15 degrees. These numbers were for gas powered go karts. I found most of this information from diygokarts.com. There I found caster can be generally set to 10 to 15 degrees of positive caster.