My son likes the games on the free website

http://www.fun4thebrain.com/addition.html

They are low key and not very exciting, but don't have timers and several don't punish you for getting answers wrong.

He particularly likes the MURB monster math game, which will let you choose addition, subtraction, multiplication, Or division. If you have a kid who plays video games often, he or she will not be impressed.