If you can’t change the maturity rating level in a profile with the Netflix Kids experience, the profile was probably automatically created when you started your Netflix account. For these profiles, you won't be able to change the maturity rating level above the allowed level for Netflix Kids. If you try to increase above that allowed level, you will get this error message:
Since this is a default Netflix Kids profile, content above a certain maturity level is restricted.
If you want to customize a profile with a new maturity rating level, delete the Netflix Kids profile with the issue and create a new profile.