Anneliese, when I switched over to all 45 minute lessons, I didn't give the parents a choice. I already had about half my studio in 45 minute lessons, but I knew the others would not do it unless I made them. For a year, every new student was not given the option. I only quoted the 45 minute tuition. I took in a few more students than I really wanted, so that if some parents got upset and quit, I would still have a full studio. In the spring I sent out a letter outlining my new policy and the very good reasons behind it. I was surprised when everyone stayed aboard, except for a few who were going to quit anyway.
You could do what Arlene suggested for a few years. Later you might feel secure enough to require all 45 minute lessons. When you do, you'll love it.
As a side note, I had a piano student quit a few years ago, because her dance studio informed her that she would HAVE to take dance several hours a day, 5 days a week. The studio said that at her level of talent and ability, that was what she had to do. If they can do it in dance, we should be able to do it in piano.