If you choose Rush zag, you might want to stay out of the water, the general cleanliness of the beach is not that good imho. The North beach is worse than the south one, but you need to go quite a way up the beach get away from the litter and bottles. Portrane which is on the opposite side of the Rogertown estuary lost its Blue flag status this year.
Do check out Donabate campsite, not sure if they have a website, but the beach and dunes there are lovely. Loads of exploration for the kids etc.