If the kid can't crush an empty pop can with one hand, he or she isn't ready to ride a bike with hand brakes. This is especially true of the low-end two wheeler which is likely to have steel rims. Steel rims are not very efficient, especially when they're wet. Combine a weak grip and inefficient hand brakes and you have a recipe for trouble. A first bike should have a coaster brake, the kind where you have to pedal backwards to apply.
Coaster brakes are efficient, rugged and responsive.
A five year old can apply enough pressure to bring the two wheeler to a screeching halt. A child's bicycle is not a performance machine, therefore the normal rules for adjusting the saddle don't apply. The child should be able to put both feet on the ground, sitting or standing. Don't expect your 9 year old to keep up with you on the trail. Don't buy technology hoping that your child will be able to keep up with you.