Well, this is a pretty old post. But, just in case, as a former math teacher in 5th,6th, and 7th grade, my main advice would be to keep it short, simple, and fun.
Games are great. Use flash cards – but only for a short period of time – one to three min. max. Work on just one set of numbers at a time, till they have it down and then go on to the next set. Use rewards only, No penalties for wrong answers. Use the flash cards at breakfast, in the car, in the bathroom, before tv or dinner, etc.
There are memory advantages to seeing and then writing down the answers. So having a bunch of pages of speed tests is helpful. But I would only do it infrequently….maybe as a way to show progress.
Remember this is just rote memory. Not looking for any kind of understanding. That is a whole different type of learning.