we have a pond and small children. it is planted with a 4ft beech hedge that has a chicken wire type fence behind it, so not noticeable but the kids can't get through it and a gate with a lock on the inside so they can't open it. I can take a picture if you like, however my garden is in a real state as we are having building work done at the minute.

At my sons school the pond has a simple stong wire grid over the top. The wire is black, so not that visible, and the gaps are large enough to get your hand through, but nothing bigger! From a distance you cant really notice it, wildlife has no issue with it. You can also get child safety netting for ponds, but that needs to be fixed very tight so that it doesnt simply sink into the water when a child falls in. This is less wildlife friendly, birds in particular can get caught in it.

On the other hand my parents have a pond and we simply never leave the children unatended in the garden at any time.