This topic comes up frequently on koi pool message boards, where we're talking about pools with a depth of water that would come well over a child's head. The sad thing is that a small child can drown in a couple of inches of water after a fall. Children are naturally attracted by water and can make determined efforts to get at it. A prevention I've always favoured is sheets of standard weldmesh. This can be cut to fit, painted black if necessary to make it less obtrusive. The advantage is that placed over a pool, it would support the weight of a child and be of no impediment to plants growing through it.