You could get a 10ft dinghy
or tender, such as a baywalker, they can be rowed or sailed, and big enough for 3 adults.
Theoretically at least, people can use tenders if they have to abandon ship in the middle of the ocean, so should be quite stable.
Though if the winds are high and you're not very experienced or there are no winds to speak of, may be good to simply use it as a row boat.
Another benefit is that if you do later decide to upgrade to a keelboat, you can use it as your tender, or boat for going to and from shore, if you are at anchor
I'd strongly advise against a laser or any other racing class dinghies
, since they aren't as stable nor as roomy. For instanec in a laser there is only enough room for your shins in the boat.
wheras a tender boat while slower, is roomy enough to carry cargo along with passengers.
In terms of the three year old, as with everyone should be wearing a life-jacket, and it would be best to practice swiming beforehand. Children are old enough to swim as early as 6-months, and at 3 they can do it in a variety of ways.
It would also be a good idea to practice capsizing and recovery, it will actually enhance how safe everyone feels, as they can overcome their fear of capsizing or ending up in the water and build confidence. You may wish to take a basic dinghy
sailing course, so can learn about the proper procedures yourself first. Though basically it is, walk off the boat when it's capsizing, when in the water ask if everyone is okay and make sure, then point the bow into the wind, pull down or even get on the centerboard to right the boat, then swim around to the transom and get in, with preferably some lines
like the mainsheet to haul yourself in, then help others in, while making sure boat stays in irons, bail out the boat, ang get underway. So that way if it does happen in an unplanned fashion, everyone will know what to do, and you can continue sailing afterwards. Who knows, maybe on a hot day, when the water is warm, everyone will be up for a swim, it certainly is one of the ways of doing so.
Also it's good to also know and practice crew-overboard procedures, which is useful not only if someone falls out, but also if something gets dropped overboard. Go on a beam reach, perpendicular to the wind for around 3 boat lengths, then tack or turn through the wind onto a broad-reach or going 45 degrees with the wind, then harden sheets to a close reach, going into the wind, and loosen them to break as you approach the overboarded on the leeward side, practice with something like a spare lifejacket.