I always suggest staying biased towards the best lens that fits your budget; a good lens will continue to be useful if/when you upgrade to newer/more sophisticated bodies but a lesser lens will never be anything but a lesser lens. The Sigma you have now sounds like a nice fast wide angle but you really need something with more reach unless you plan to run along with her and I doubt if your daughter or the officials would smile upon that 🙂
With running I bet you can do a pretty good job of keeping a single AF point on your daughter but that isn't absolutely necessary. The cross point style AF sensor provides the highest accuracy of the phase detection sensors available on you camera HOWEVER it isn't necessary to activate the cross point sensor (or that spot's cross point capabilities) to achieve excellent focus. My 1DX and 1DX2 bodies don't have cross point capability with all of their selectable 61 points and which can function in cross or dual cross mode is further limited by the lens so don't discount the functionality of a "standard" AF sensor point. I shot a lot of soccer and track photos of my daughter with my Canon 1D2 which only had a few cross point sensors in the middle of the AF array and I have some great photos from those years before I upgraded to a 1DX 2.
For running (true of any sport) your placement is the key to getting the best photos. You want to fill as much of the lens image as possible with your subject while keeping in mind speed of approach/departure which impacts how fast your lens has to follow focus. A panning type shot keeping your daughter's image sharp while further motion blurring everything else is another way to make a runner pop out and create interest. It takes a little practice to nail this shot and her practice sessions (if they are open to the public) are a great time to practice these and other techniques so that you will be ready for the real events.
When my daughter was running track (primarily 100 and 400 meter individual and relay events) I mainly used a 70-200MM 2.8 lens which is very versatile for events like these. She primarily plays soccer now but is taking up cross country next Fall to stay in shape during the soccer off-season so I will have to rethink my setup for those events.
But the most important advice is to get plenty of the best shots you can now because these years pass VERY quickly!
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
