Timers are an amazingly simple tool for helping kids focus on math or other homework. Their best uses are for:
A timer doesn't have to be fancy. Just the act of starting a clock is enough to focus most kids and make them want to "beat" the time. Here are the ones I've had the most luck with:
Sand Timers: I know they are old-school, but maybe that's why kids like them so much. The falling sand does a great job of keeping kids focused, and you don't get any annoying beeping when the time runs out.
The Miracle Cube Timer: This ingenious timer cube is as simple as it gets. Kids just turn the cube to the time they want, the clock starts automatically, and an alarm goes off at the set time. Wow!
Simple Stopwatch: If you want to go with a traditional stopwatch, make sure it's super straightforward for kids or you'll have beepers going off all the time. This one is a good choice; it has buttons for "stop", "clear", and "go". That's it.
The Timer On Your Phone: In this day of smart phones, you may not have to buy a timer at all. Your phone probably has a stopwatch feature on it already that your child can use. Of course, you do have to be willing to surrender your phone whenever your child needs a timer...