hi Catja,
i live and run my business here in punta cana 12 years now, february to end june is our quiet time with the weather, of course you can get a rainy or windy day all over the year on ever spot of this world, but you can expect beautiful weather conditions to let your 8 year old going to swim alone. the most quiet beaches for kids you will find in the areas south of El Cortecito, hotels named Paradisus Palma Real, Melia Caribe/Tropical, IGFA Villas Bavaro, Barcelo Bavaro Beach Resort, Natura Park, Occidental Allegro Resort, Sunscape Punta Cana Grand, Catalonia Resort, Club Med and Punta Cana Resort & Club. for diving we are sorrily not a top destination of this world, if you and your husband are interested in watersports top activities you should try offshore Big Game Fishing, in february mostly for Dorados and wahoo and Sailfish, we are one of the caribbeans number one spots for that activity. our reef is very close to the beach, but of course you can swim in the ocean at any hotel on the east coast.
weather conditions for the Bayahibe area on the south-east are during that time of the year similar to the ones at punta cana, there are some nice diving spots close near Catalina island, but don't expect the big deal if you're an experienced diver who dived the top spots.
the above mentioned Melia Resort has besides it's location on our most beautiful beach a great Kiddies activities program, so your 8 years old would be in best hands there.
enjoy paradise island
Mike