It uses adobe flash, so if your child will access it on a computer, make sure it is enabled on your browser. Because some of the 400 games uses adobe flash, not all of them are available for play on other devices via the app on their device but still over 300 games are available.
It is free but there are also subscriptions available with added benefits.
I also saw this article on cnet with some educational game suggestions.
I hope this is helpful to you and your children.
Until next time, God bless,