Airport express (AE) pass validity beyond 3 days unlimited MTR ride

I am planning to be in HK for 7 days and planning to buy the 2 way AE pass.
1. If I use the AE card on day 1, can I still use the 2nd "free" HK to airport trip on the 7th day?
2. Will the 3 days unlimited ride start on the day I use the AE between airport to HK? or can it start on the 2nd day (assuming i am not taking any MTR on day 1 other than the train between airport to HK)?
3. Is there a kids price for the airport express pass? They are aged of 3 and 5.