What should children know?

I have two ideas on this topic of knowing terrible things as a child. I feel that kids must know things at a certain age because knowing at a older age can bring anger to a relationship. Taking it from my own standpoint if I were adopted and I didn't find out until I was much older I would feel like I was lied to and feel very sad. I would appreciate what my adoptive parent had done for me but I would like reach out and see my family. One exception to this is if my parents are messed up and really can't take care of themselves. I also feel that there is stuff kids should never find out an early age. I have a story of something children should not find out but most children eventually find out. About 10 years ago my brother had lost a tooth. My brother and I came up with this crazy idea of staying up all night to see if the tooth fairy was real. So we stayed up and were shocked to find out that the tooth fairy were are parents the whole time. We were shocked but also thought it was funny because we actually came up with this crazy plan. We decided to keep our discovery to ourselves because telling my dad would result in no money when we lost more teeth.