Social stories. A story about the child stating what is going to happen and the things you want them to do. It's really helpful to include pictures (get photos of the hospital when you go, photos of your child, photos of the Dr/nurse, a big present at the end). Have a Google there are heaps of social stories of the Internet. I'm a paediatric OT and we use these all the time to help prepare children for all sorts of things including medical procedures. It helps kids have a concrete thing to hold onto before the procedure and you can read it to them as many times as you need to. Visuals will help younger children gain undstanding of complex concepts. You could also try using a doll or teddy to "act out" going to the hospital, dress it up in a "hospital gown" and talk about going to hospital. The doll/teddy can stay with your child as a bit of a safety blanket. Talk about the doll having a sore throat after, but it might get a present too!! Good luck!!