Ventilation and light are the best solution: you need a powered extractor fan though, ideally connected to the light switch so it always comes on. If you can make the place brighter, that will also help. Have you checked your silicone sealing? Cheap, worn silicone is great for fungus, you need to use the correct one, containing a fungicide. Finally, repaint with mould resistant paint. Actually, forget all that logical stuff, use fear on them! Several household mould spores are potentially quite harmful, especially to children. When you have a shower in a mouldy, unventilated room, the hot, humid air carries the spores around and into your kiddies soft pink lungs. Do it for the children!