There are lots of ways to teach kids to focus. One is to use a Timer (you can use a kitchen timer, watch, phone, or timer app!). Have the child play with something (preferred) for 3-5 minutes. They can't switch activities until timer goes off. If they try to leave bring them back until the timer goes off. Then they can switch activities! You can also do various other table tasks in between taking breaks such as mazes, dot to dots, coloring, worksheets, puzzles, games, etc to work on focus. Do each activity for about 5-10 minutes (depending on child's age and ability) and then take a little break
Recommended apps: "Countdown", "VisTimerFree" for iphone.
You can buy timers specifically for teaching children with ADHD or learning needs at www.timetimer.com