Nick - congrats on getting something out the door. We have a nanny and I don't think we would use this application for her. It's possible that we are in the minority, but we have had her for almost 5 years and she knows what needs to be done. It's pretty rare for us to ask anything out of the ordinary - certainly not prevalent enough to require an app to manage it.
HOWEVER - as a family, we could ABSOLUTELY use something like this. We have a big family - and a lot is expected of each of us. We have kids that aren't in school yet, and kids that are in high school. Keeping things straight is a real pain in the arse.
My wife should be able to view my schedule, or a family schedule, etc... Each member would be responsible for updating their calendar. Todo's can be tasked to whomever - and tracked - just like reminders. Heck, I would even like to see a chore-chart type thing in an app like this where it creates some todo's based on recurring chores.
So, this doesn't really answer your question - but maybe it could expand your market a bit!
To answer your question: How about a different approach. Our nanny doesn't have a smart phone - so mobile or iphone/android is out of the question. But she can get sms messages. How about making adding sms support to what you already have using a library like twilio to support uni or bi-directional updates/notifications? Nanny gets notified and can reply to txt or whatever but the website is always available for additional interface/views/etc...
Well, just an idea. If you expanded into the "family" concept, I would definitely recommend an iphone (smart phone) approach over mobile web because of the enhanced notifications and alerts and whatnot offered by native apps.
Hope this helps a little.