If they get some advertising going, that 500k in sales by the end of the year would be easy cheesy IMO..
You also have to take into consideration the uniqueness of the games. There has to be well over ten thousand children with birthdays that fall into the 3 to 8 age range every day in the US and Canada. The combined population of two countries is somewhere around 340,000,000 and the games are in English, Spanish and French.
@ 10,000 birthdays a day for the 3 to 8 age range that is only 3,650,000 kids in total for a year. I am guessing the number is much much higher than that..
You are correct though, the key right now is meeting supply while demand is high.