Can you drain the pond and then have it easily re-filled by its source water?
Hand-laying cobble size stones would probably be a good solution to make a "stairway" out of the pond. However, that is also a lot of work.
Dumping a load of cobbles would be easier, but those stone are typically treacherous to walk over when we cross streams in Colorado. (Either way, you need to put heavy landscape fabric down first, so your stones don't sink into the clay.)
For a very steep slope would you consider just making a normal set of stair stringers with pressure treated wood? You can then also make the treads with pressure treated wood, or plastic decking material with grip strips added to combat the slippery slime. You would need anchors to hold it in place. I would also make a handrail on one side as the only portion that would stick up above the water line. That way every swimmer could find the stairs and there wouldn't be an eyesore in your pond.
Another option would be to just build a dock (either floating or on piers). You could hang a vertical steel ladder off the end of the dock. You could also make a brushpile on the side of the dock opposite the swimming area to enhance the fishing. I like putting a life jacket on small kids and fishing from a dock. They are easier to keep under proper supervision in that manner!
Have fun on your future family activities at your pond.
