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. See rule one. Learning to cycle with mom and dad is much like learning to walk.
Bicycle tires have a pressure rating stamped right on the wall.
Some tires indicate a minimum and a maximum tire pressure. A soft tire picks up little bits of gravel and glass off the road, these get imbedded into the tire, and chew away at the inner tube. You often see people squeeze the tire. Squeezing can tell you if the tire is too soft, not if its hard enough. You need a proper air gauge to check the tire pressure.