Well, aside from the obvious 'never letting them out of your sight' thing.. you can get very sturdy metal grating to put over the top - my mum was a registered childminder and the council was fine with this being safe.
And you can get something similar (but more expensive) that sits just below the waterline so keeps the pond looking pretty, though it still means there's an inch or two of potential danger. Otherwise you could have a bog garden?
Melanie