There is no single answer to your question. The best lens "for taking sport shots" of your boys, or anything else for that matter, depends highly upon your shooting location: indoors or outdoors: how close you are to the action; how "tight" do you want the images to be; etc.
Basically, you want a camera with as many AF points as possible, a high frame rate per second, and high MP is not as important as other factors like high ISO performance and high shutter speeds.
One of the best all around lenses for shooting sports are the 70-200mm lineup of Canon "L" lenses. You also would want a "fast" lens, which means one with a low f/stop number. You want something f/4, or better. Canon DSLRs perform better with f/2.8, or faster, lenses, by design.
On a budget? I think the EF 50mm f/1.8 STM, or the EF 85mm f/1.8 are good choices on a crop sensor body camera for shooting fast action. The low f/stop numbers will give you better DOF, depth of field, because they will exhibit lower hyperfocal distances. In other words, on focus setting will work if subjects are 50 feet away, 100 feet away, or more.