This is just my opinion as a parent who helped fill out the CSU app last year at this time with his D21.
I believe they’re looking for paid and/or unpaid work experience in the fields of CS or Aerospace (or whatever your child ultimately picks on the app). And being a math tutor would not be major-related because CS and Aerospace jobs use math in their fields of study.
Math major, yes, being a paid or unpaid math tutor in math is major-related.