Hi Ariel, and thanks for bringing that up! I have gone back into the Masterclass and have found what you are referring to.
There is a section (WHO masturbates and how this behaviour changes as your child grows up - What is developmentally appropriate behaviour?) that talks about the sexual behaviours a child may display as they grow up.
I have used a European tool and they explain that children may want to masturbate with others, and if they do it is seen as being developmentally approriate behaviour, as long as they are only masturbating themselves and not touching other people's penises.
So... this is where it starts to get confusing, as just because a behaviour is seen as being age-appropriate, it doesn't mean that we let them keep on doing it. We would discourage it and ensure they know it is a private thing (not public).
I'll jump into the Masterclass and add a note into this section of the readable notes.
I hope this makes sense! Cheers, Cath