I used to drag my family of wife and two kids under 15 around the southern bay in a 4.2m runabout, similar to what you are looking at. We'd also drop into places like Horseshoe Bay to stretch the legs and let the kids have a run around etc. We enjoyed the runabout style with windscreen and comfortable seats for the adults. Kids don't really care about their seats and backs at that age, although I did get a cushion for the rear seat.
That boat had a 35 and then a 40 horsepower 2 stroke motor and was more than adequate up to around 12 knots. After that you'd have to slow the trip down a bit or preferably not go out far. The more south you go the better the protection by the islands from the winds.
I agree with going the larger boat theory. 3.5 to 4 will see you outgrow it quickly. Even though I had a 4.2 I always thought something a bit bigger would have been better. The truth is a bigger boat would have been better. The 4.2 was definitely the minimum for what we did.
I now have a 4.7 with a 60hp 4 stroke and it opens up a bit of the northern bay, so on a good weather day I can get to Moreton pretty comfortably and enjoy places like Tangalooma. Something more for you to consider.
As they are phasing out 2 strokes in the short term (very soon I believe) I'll assume you will be getting a four stroke. That is a good thing as your range is far better. I'm averaging a bit under 7 litres per hour (about 5.3 km/l) with the 4 and was getting about 2km to the litre with the 2 strokes.
So in summary I suggest -
- Get a tinnie at least 4.2m. (The tinnie will also be more forgiving of hull damage at the ramp and on sandbars etc while you're getting used to boating) subject to the size of and age of kids and how many there are;
- Get a 4 stroke;
- Get a runabout style; and
- Get as big as you can afford/tow.