A kid above age 4 should shower everyday as they tend to get dirty and can begin to smell even if you don't notice it.
Although it's okay to skip a shower once a day every so often.
Unless the child gets dirty then they of course should shower or bathe to clean up.
A child can bathe or take a shower by themselves by the time they are age 5 or 6 years old.
However with a child even though they can take a shower by themselves may need help or supervising to ensure the child actually bathes or showers and gets clean.
Some kids may not actually bathe or shower when you think they are doing so.
But at age 5 to 6 years old a child can most often easily bathe or shower by themselves without any assistance.
I started showering and bathing myself when I was 5 years old.
However my mom would still come into the bathroom and check on me and help me wash my hair and back as well.
But other than that I could shower and bathe myself without any issue.
Some kids may not be ready to shower alone until they are 6 or 7.
It depends on the child but you can begin letting your child bathe alone or shower alone by age 5 to 6 years old.