Justin, I agree with Dan. I have 3 kids and their immunization needs are kept by the physician and he lets us know when they are due, along with the school. That said, if you used the same kind of model and set it up where a parent could enter all their kids scheduled activities and it would generate an email each day that includes the what's on tap, that would be useful.
For instance, if I could input my son's baseball schedule, my daughter's tennis and dancing classes, and my other daughter's track obligations, and then receive or access a daily list of who has to be where, I think that would be worth using. The immunization aspect could also be integrated in it. I realize that there are scheduling programs to handle this, but if it is web-based, it could be accessed by anyone with the login and password. So grandparents or extened family could view it. Just an idea....