Children under 12 must always wear a lifejacket while in any boat. It's the law. Make sure that children are supervised when swimming. Lack of supervision is the most common cause of drowning. This is every bit as dangerous as letting them play on a freeway, and the proof is the number of kids who end up in area hospitals.
Bacteria are present in all lake-water.
Do not let children wearing diapers go into the lake unless they are wearing leak-proof plastic pants. Do not swim in the lake if you have had a fever or experienced nausea or diarrhea within the last 48 hours. You can spread disease. Do not feed the ducks and geese. Their droppings are the cause of swimmers itch, and they cause fecal coliform contamination of the lake water and foul the beach areas.