SBM
Could a large enough body of water (natural or manmade) been partially frozen, Serenity falls in, then it freezes solidly once the temps dropped and the snow thickened? Ice is very heavy and will hold a child's body down if the mass of ice/snow weighs enough. Cans, barrells, buckets, holes of any sort, swimming pools, dumpsters, abandoned truck beds, farming equipment, etc., basically anything in which water can accumulate and freeze should be checked carefully. Keep in mind that thick ice will not be as clear as water would be. I'd suggest taking a careful look at any large enough ice masses besides just ditches and ponds.