The Minimum and Maximum Grade Level fields are available on the Registration Config tab on a program class record.
Program Classes have a Minimum and Maximum Grade Level field on them. This allows you to filter classes to only apply to certain grade levels. However, if a person has no grade level associated with them (like a parent), all classes offered within the Program Registration form are available.
If you need to differentiate between which classes parents can register for, we recommend using two different Program Registration configurations. You can read more about configuring Program Registration courses and classes in documentation here.