All of the points above are good points and I agree with buying a 60 size trainer. It lands slower and that tends to be the most difficult thing to do when you start IMO. I would recommend going to a flying club and getting your hands on a buddy box. I would not recommend putting an instructor through teaching all 3 of you to fly at one time. If there are multiple instructors then it may not be a problem. It is rather difficult for an instructor to teach all of you at once simply because the instructor will want to get in some flying time of his/her own also. Not to mention it can be stressful.

I'm glad to see that you are getting your family involved. My wife flies and I am teaching our 10 year old to fly. My 3 year old can't wait, he can already do nearly perfect loops on a buddy box of course. And because my family is involved, I get more planes. YESSSS. The younger the kids are when they start, the better off they are... We have young teens in our club that can fly circles around the more experienced pilot. Only think is that their planes have only one speed.

Lot's of luck and I would recommend that the twin be put on hold for now.
Just my 2 pennies worth