I would not introduce my kids to Farmsatoshi because all indications are that they are just another Ponzi scheme. See : ChickenCoins
Yes, you are right on using Piggycoin as a example and more gaming related sites should catch on to this.
Let's just hope, when they make tons of Bitcoins, they will not use it on Silkroad III. ^chuckle^
A Ponzi scheme is where you invest money and then get a return on your investment, only it turns out they pay you back with other, later, investors money.
Farm Satoshi is a game that pays you, in essence, for viewing advertising. You can't invest money into it, so it cannot be a Ponzi scheme.
I'm at level 20 in the game now, and it looks like I'll be able to earn about 100,000 satoshi/day after I finish power-leveling. I've got 550,000 satoshi laying there right now if I wanted to withdraw it. And not a satoshi invested. ;-)