I welcome anybody to correct me on this but in all the reading I've done (allergies aside) I haven't found any actual evidence of spalted wood causing any more health problems than clear wood, aside from the occasional folk tale of mushrooms growing in someone's lung. We all know wood dust is a hazard, spalted or not. I don't hesitate to use spalted wood for salad bowls. I just don't see how a rogue oil soaked spore which is specifically designed to live on wood could be any more hazardous than all the spores we encounter all day, every day in our normal environment. Admittedly the lung mushroom thing still nudges me toward being a little more careful about breathing dust from spalted wood, but I really am not sure there is a scientific basis for it.
All that being said, infants may not be the best test subjects but I'd chew on that rattle without hesitation if that was my thing.