A soccer field is rather big. Where are you in relation to the dogs?
- The pics make me think you are on the field, not up in the bleachers.
Upload a few uncropped images. This will let us see how much "space" there is around the dogs and handlers.
- The more space you have, the longer the lens you may want.
- BUT, a long lens can be harder to track a fast moving subject. Everything comes with a compromise.
If you are on the field a GOOD 70-300 may work fine.
A 100-400 will give you more reach, at the cost of not being wide enough if they get closer to you.
If you are up in the bleachers a 150-600 will work better.
- Warning, with something like the 150-600, you are going to need to use a monopod or tripod. It is bulky and heavy.
There is also the issue of technique.
A 300mm lens on a T7i is a 13.7x magnification lens.
That requires proper technique to use well.
For a moving subject, if the shutter speed is not quite fast enough, you will get subject blur.
I would be up at 1/1000 sec.
On the T7i, if you are using "sports scene" mode, the autofocus is in zone mode.
In zone mode the T7i uses "closest subject" logic. So if the handler is closer to you than the dog, the camera will focus on the handler, not the dog. It could also focus on the grass between you and the dog (I've seen that happen once in a while).