Probably none. You do have a responsibility, if you have a playset, to use reasonable care to make sure that the set is safe. This means that the equipment is safe and constructed according to the manufacturer's directions. It also means that you care for the playset and ensure it remains safe for general use with no visible hazards, like overhanging nails or loose metal or wood. However, you are not responsible for a child who injures themselves when they play on your playset without your knowledge or permission.
If a child falls and hurts themselves, then maybe some coverage for the medical bills available under your homeowner's medical care payments policy. Check your homeowner's insurance for more information. As far as lawsuit are concerned, however, you are only liable to someone if you have been careless.
A swimming pool is only slightly different. In addition to your duty to use reasonable care to make is safe, you have a duty to comply with your local swimming pool safety code. For example, say the code calls for a six-foot fence with childproof locks and you build a four-foot fence without locks. If a child sneaks in one day when you are not home and hurts himself, you probably would have liability for a lawsuit.
If you follow all laws and safety guidelines regarding equipment or structures on your property, it is unlikely you will be sued if a child comes onto your property without your knowledge and hurts themselves. However, if neighborhood children are frequently coming to your home, it is a good idea to speak to their parents and make sure they are aware.