Why would you give an iPhone to your 2 year old?
I can tell you that multi-touch has been a lot easier to teach than mouse use.
Will be interesting to see what the future holds - perhaps the idea of a separate mouse to a screen will be seen as quaint one day.
The funny thing is the mouse should be a a very basic instrument - its basically point and grunt. But the dexterity required for kids is a bit of a step - touch seems to make it even more intuitive.