In our situation, I will be travelling to Florida with my son. My wife and daughter will be travelling down separately and hooking up with us later that day (I saved 50,000 aeroplan points doing it this way)...
But as a result, both of us will have to travel with permission letters stating that it is ok with us that the other parent is travelling with our child to the US without us... I give permission to my wife to take our daughter to the US and my wife gives me permission to take our son to the US...
Funny huh?
p.s. We'll be getting passports for the whole family too, just to be on the safe side.
Mike in the Maritimes.