Very frustrating Pete , in fact really annoying, and not an "abnorma"l thing to deal with.
Greeks rent a part of the beach, from the local council, for a place not only to keep tidy but also for rental of the sunbeds etc to also make a few bob.
Unless the kiddies were making a sandcastle smack in front of other empty sunbeds, which potentially costs the "renter" money then I, for one am puzzled as to why he complained.
At the end of the day the beach is the beach and, providing, no one is interfereing with anyone elses enjoyment then you go and do your, or your childrens, own thing.
You sure it wasn't you doing the sandcastle? Us adults can get carried away sometimes
( I've done some good uns in the past. )
I hope it didn't put too much of a dampener on your hol.
Cheers
Negg