For little girls, sitting on some folded toilet tissue as they get off the toilet is often all they need to clean up after urination. A lazy-susan is a good spot for holding liquid soap in a pump bottle, toothbrushing needs, etc. Follow your child's lead and allow them to do all that they wish to try, even if they are extremely young and don't do a good job yet. Double shower head systems are sold, where a regular shower head stays in place and a diverter button sends the water to a hand-held shower, so even a very young child can help with bathing and hair washing. Using this double-shower head, shower stalls are also very useful, but it's important that the water controls be within the older child's reach.
Grab bars are available for tub sides and walls and are great safety items at any age.
For older children, metal legs on taller bath seats can be cut down to size. If necessary, all these items can be bent with a heat gun to suit the child's reaching needs. There are numerous gadgets available commercially that have long, extended handles to make reaching easier. Different versions have brushes, combs, and bath brushes. Such items can be found in medical supply catalogs and are useful for regular hair care and shampooing, as well as bathing.