I hear ya, kids running about can test the measure of an AF system alright. The Fuji 16-55 F/2.8 WR is very fast, for that level of hyperactive swerving kids can manage, although a little heavy + $$$.
The same reason is holding me back too, the ISO. I'm fine for Fuji's ISO 6400 most of the time, I think it is very detailed and not so much colour noise, more of a grain feel. I did have a preproduction X-Pro2 for a little while & ISO 12800 was close to the now four year old 5D3, the joystick was familiar and nice to use, the shutter lighter to push and the speed on the whole felt faster, I'm not sure I'd really say it was very noticeably faster in AF to the X-T1 as I'd need to do a side by side test of something fast like kids in the playground. I also have an itch for an A7R-ii/A7S-ii for the higher ISO, just trying to decide if I really want to go down the long and winding Sony road or buy into more Fuji. Perhaps I'll swap out the 5D3 for one [Stabilised 135L, yes!] or sell it all and settle on Fujifilm.
To date all my clients have been really happy with the shots, including the ones for publication, no matter what camera they come from so I guess it is just my feelings holding me back, that and the dreamy 2/135L. When I look at the images after they have been exported out of lightroom, I honestly have a hard time telling what camera took what shot.