I 2013 there was a conflict in the Danish primary school. In order to keep the kids active I thus created a game in which the kids could practice calculation. The result can be found here (Danish site): LINK
The idea is that kids can practice mental arithmetic. Depending on your success, it gradually becomes more difficult. An extended explanation (in Danish) can be found here: LINK
In order to play the game you must create a user, and be logged in.
If you want to host a site yourself, you can download this file: LINK. In the file you find instructions about how to install it (in Danish — but Google translate might help).