The scientific explanation is "surface tension". This is what is called a "tensioning force" which occurs on the surface of a liquid because the molecules cling to each other. A nine-year-old's version might be to tell Lorna that it is bit like a very thin skin on top of the water (but stress that it is only like a skin, not an actual skin, otherwise scientists get very cross). Small objects can be held by this skin effect, so isolated grains of sand will float, while clusters will sink. Then mention pond insects that walk on water because of the same effect (and stand by for another barrage of mind-bending questions!).
If your pupils have asked a tricky question, please send it to Diane Hofkins at The TES