A favorite beach for locals, Baldwin Beach Park has tide pools, swimming and snorkeling.
We were staying in Pa'ia, and this was really the only beach in the area that we could find that would be good for kids. This side of the island is great for surfers because the waves are much bigger (epic in some cases!).
This is a nice flat beach, with lovely sand. It is one of the beaches that has lifeguards, and is just outside Pa'ia (just before you come into town, it will be on your left) so you have the advantage of food and shops being right there (including one of the best food stores on the island, and some would say in Hawaii, Mana Foods). My husband found the water too rough for my son to swim in (he is 3 and was not comfortable with the size of the waves). They did go in the afternoon, so wonder if the sea would be calmer in the morning. The boys spent their time digging and building in the sand instead.
This beach has bathrooms and showers and a decent size parking lot.