Sopot is the place you need to go, very small place, extremely safe. ... and a beautiful beach for kids to play on all day.
Note that during the later part of the summer, the water is often out-of-bounds due to algae. The water is very quiet, gets very warm, and then turns red and icky.
We've given up heading for the beach there and head north instead -- either the north coast proper, or somewhere on the north side of the Hel peninsula.
Chałupy, on the Hel peninsula, gives easy access to both the Baltic and the Bay of Gdansk.
villabaltic.pl/ : never stayed there but know the people who own it and they speak fluent English.