First I must say that it's good that you think about the safety issue before you start building the pond!

Wood submerged in shallow water will decay. Wood preserves only in very deep water, with very little oxygen. I would also advise against using wood under the ground, for the same reason. I think there are treatments that will slow the decay, but I don't know how effective they are and I know that I wouldn't want these materials in my back yard where my children play.

The most common solution is grilles or meshes made of plastic or steel. Take a look at the advice part of this page for more information.