Although all stings can not be avoided, many can be. Simple steps can be taught to young ones and can help prevent a miserable beach experience. Swimming is fun and should be encouraged.
Stingrays lay on the bottom, partly burring under the sand. When stepped on, they will whip their tail up and stab the offender with their barb. It is a defensive behavior. Always shuffle your feet on the sand bottom when wadding in the water. This way, you will alert the ray of your presence and it may scare them off. Teach young ones to shuffle their feet and never "walk" as if on land.
If you find a beached jellyfish, do not touch. They can still sting. Often turtles will eat jellyfish, and some tentacles may break off and find their way to the beach, and they can still sting. There are products that claim to prevent jellyfish stings from occurring and products for treatment after a sting, which are basically vinegar solutions suspended in a jell and stick on the affected area longer.
Jellyfish usually travel in groups and are not fast swimmers. They are carried by currents and the wind mainly. If you see them in the water, simply find another place to swim. If you do get stung, park officials often have had experience with the treatment and may be able to help you, so if they are around, get help. It is always a good idea to have some white vinegar in your beach bag just in case someone experiences a sting.