As mentioned, as July & August approaches, the best time to swim, you're going to find that the nettles that far south will be a real nuisance and sometimes worse. (Thats one of the primary reasons we chose to keep our boat North of the bay bridge)
Consider some other things you can do as well, for those really warm days we have a large storage container we put in the cockpit and fill with water from the dock prior to departure. (if we use bay water to fill the container I add bleach in proper ratio). Kids fishing rods, you never know, they might catch dinner. We had two little play boat attached to a long string, love letting it and pulling it in for what seemed hours. We also rigged a swing from the dodger...all fun things for them.
At 4 yrs old, they are probably to young to start sailing, but the 6 yr old could start doing some things like taking the helm, read the charts and try to figure out the location, learn knots (good practice for me too!), a bird book to identify, etc...
Although we have not done any extended cruising, our longest day for us was a 14 hour sail from the St Marys River to St Michaels or about 90 miles IIRCC. Longest we have been without touching land was four days on the bay...never have they been bored or felt the need to get off the boat, more the Admiral then them LOL Maybe we're just lucky, but I believe it has a lot with how much you choose to involve them in the art of sailing and the cruising life itself, which it sounds like your doing all the right things