The grandmother could have argued legitimate interest (she is a grandmother and took care of one of the children) and have demonstrated limited visibility (now there was no mention of posting publically or not, so the judge went with publically) and the case would have started to get interesting.
Now it was just a slam dunk. No permission, no argument for use case under GDPR, done.
[1] https://uitspraken.rechtspraak.nl/inziendocument?id=ECLI:NL:...