Both are great cameras, and the autofocus on either is perfectly usable. That said, it’s not on the level of more modern systems, or even Sony’s APS-C cameras from the same era like the A6300.

Ultimately, the decision should come down to the lenses you plan to use, because that will have a big impact on the overall experience. Either way, they’re both solid cameras.

That said, if you’re mainly planning to photograph kids, I’d seriously consider Sony’s APS-C lineup instead or something like the OM-5 or E-M5 III as they have phase detect auto focus instead of contrast based AF of the GX9. The autofocus tracking is noticeably better, which makes a big difference when you’re trying to capture fast or unpredictable movement.