From personal experience with my own "niche" market for this age group, I would say that the two times a week schedule will probably lead to resistance by parents or an inability for some of these already overscheduled children to attend or enroll for lessons with you. Also, 30 minutes is really, really short for a group lesson length for this age group. I would encourage you to consider a once-a-week 45-60 minute length lesson instead.
Are you talking about a general preschool music course or a dedicated piano course? That makes a difference as to the success or lack of success involving parent observation of the lesson. Which leads to another point: There is a difference between parental participation and parental observation. Often it is a very fine line that means you really need to develop a philosophy of boundaries for how you handle it. And the curriculum you choose should really have a bearing on what you utimately decide to do. Good luck