[Samba] valid users and file permissions

[Kyle]

Hi people,

I'd like to understand valid users and file permissions better. I have a 
share which is not behaving as I expect.

[family]
path = /home/shares/family
create mask = 0664
directory mask = 0775
force group = parental
guest ok = No
valid users = @parental, @family
writeable = Yes

in Group parental are mum & dad; in group family are mum, dad and offspring.

With file permissions of 0664 and force group parental, I would expect  
the offspring to be able to browse the share but not write to or delete 
from it. Unfortunately, they can both write and delete.

How do I achieve this please?
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Kind Regards

Kyle