With winter pool covers, inground pools are protected from debris getting in, and from loss of water to evaporation and from loss of winter pool chemicals. What they don't do is to actually protect a child or pet that chooses to walk on the cover. Regular winter pool covers aren't made to be strong enough to take that kind of weight. They also don't have what it takes to help your pool retain heat – the way solar pool covers do. They do exactly what they say – protect your inground pool from dirt and losses to evaporation.
They make winter pool covers out of either vinyl or plastic mesh. Both these types of material happen to be UV resistant. If they weren't UV resistant, you'd have them cracking and breaking up pretty soon in the cold and in the sun. Buy the better quality ones, and you'll find that they make them out of them multilayer vinyl. This material happens to be stronger and usually, it resists algae growth is well.