One could always need a spare.
It sounds crazy, but I've currently got 5 floating around, and I'll need 2 more when they finish rolling out. A couple were free from GO (you could trade in your old book of tickets for a free card, with equivalent rides).
There's mine, my wife's, one for the 9 year old, and one for the 5 year old (mostly just to open subway gates at this stage - but he was very excited about it). And a spare, if I'm out with someone from out-of-town or something.
But when (if) they get to the stage where kids really DO have to tap every ride ... I'm not really sure how we would keep passing the cards back and forth - it will be a nightmare. Probably easier just to have 2 kids Presto cards each - otherwise disaster will surely ensue.
And somehow I expect that when my MDP pass goes Presto, they'll just end up sending me a new Presto card whether I need it or not.
I do have to say, the ability to manage them all in one account is great! Though they all have autoload going - except the 5-year old. Which has been painless for years.