O.K. So I've been following along with this thread since yesterday. First, I would like to say good luck to the guy who started the thread(jackofclubs I think). Second I would like to say that children will not sit still to an hour and a half show that has not had enough time put into it.
Children are allways looking to be entertained, and if you lose their interest, and the parents notice, then you'r show is done, and you'r reputation might go up in flames. The half-an-hour show is good enough, an hour and a half will be too much in a days time. I don't care how long you've been doing magic, no-one has the ability to create an hour and a half show in one day. The largest contributor, is that you have to practice, and make sure it will last that long. Twenty four hours in a day.... I don't know about you, but I have to go over my routines countless times before performing them. And at an hour and a half per full practice routine, I think you would get pretty tired and not get much proper practice in.
I have a four hour walk-around gig in my town next weekend, and I have put literally tonns of time and money into it, but I have had a month to do it. If someone told me they wanted me to make it 12 hours now, I would say no, not because of the time, but because of the time I have left to get everything together.
I think most people on this thread will agree with me and say, it would be really hard to do you'r job efficiently enough for the parents, and children. If I were you, I would tell the lady that you just don't have enough material to cover an hour and a half, rather than trying to tell her that her kid won't sit still that long.
But again, the best of luck to you, no matter what you decide.