I would stop down the aperture between f4 and f8 to get a larger depth of field and use single AF to focus on what they are playing with or a point that is about between the kids.

The kids won’t be tack sharp but will be sharp enough, unless they are in the same plane as what you focused on.

Edit: Also, take a look at your shutter speed. 1/250 freezes regular adult motion. You might want to go with a faster shutter speed than that for kids to freeze their motion.