Andy,
While it is perfectly understandable that you want to take your family flying, I can't help feeling that you are pushing it a bit to take them all together for their first flight in a small aircraft. Presumably your wife will sit in front with the girls behind you. If, for example, one or both children become distressed by turbulence which they don't understand, your wife, and therefore you, could become seriously distracted. Having any passenger distressed can be very distracting and when it is your own family it seems a lot worse. I was lucky when one of my boys became airsick, I was grateful to have an instructor with me which helped to keep the pressure off me until I could land. Incidentally, I was only in the circuit at the time.
It might be better to take the girls up with an instructor (or experienced pilot) with you to see how they get on. If they enjoy it, they will want to go again and will encourage your wife to join in.
But don't underestimate the power of motherhood - my wife flew with me before we had children, never when they were young, and, now I'm getting on a bit and she is only too aware of how fallible I am, she only flies happily when there is another pilot present. The thought of being stuck in an aeroplane with an incapacitated pilot terrifies her. Add children and she would probably panic with the first bump. With another pilot, I have flown her through some dodgy weather and, as long as there is some sun or a shop/restaurant at the other end, she is quite happy.
In sailing and flying, once you put wives off, it is very difficult to restore confidence.