DS was also about 4 when he asked, "I can understand how the mom's genes get into the baby, but I don't see how the dad's get there." So I told him; I've worked in reproductive health a lot and am pretty fearless about talking with him about anything. He thought my explanation was disgusting. But, he was also reading all the movie reviews, and had noticed a lot that said "rated R for sex." So he was really glad to know what sex was and felt confident that he was not missing anything interesting by not seeing those movies! I'd rather have him hear if from me than anyone else or try to figure it out on his own.
Re: Tooth fairy. When he started asking, I just kept asking him questions back, like, "well, why does't it make sense? What would explain it, instead?" I let him figure it himself out with some coaching from me. he was so proud when he got it.
My mother always said that if a child is old enough to formulate a questions, they are old enough to get an answer. Start your answer at a basic level and then go deeper if they still seem curious or ask more questions.
