I think the best place to start is by making sure there are as few tripping and slipping hazards as possible around the garden. Kids like to run and play, and while they’re typically quite good at recovering from a hard fall, it’s still better to try to prevent as many falls as possible.
Potential tripping hazards include toys left in the backyard, decorations that are in the way, loose rocks hidden by the grass, etc. Any wet surface can cause a slip, but you should be particularly worried about wet surfaces around a pool or a pond since a kid can slip and fall into the water feature.
Try to keep those areas dry, and consider installing a cover for the water feature in question. There are many kinds of pond covers out there, as this website shows.
Finally, make sure to remove any gardening tools and other supplies you left outdoors. If you have a shed, keep it locked.