I believe you can choose to be an exit relay or a middle relay. A middle relay only connects to other relays, it does not allow "outgoing" connections, i.e. you won't appear to be browsing child porn from the outside if that's what you mean.

If you mean that you don't want to relay traffic that carries child porn, than you can't do that, since you can't see what you are relaying (and even if you did, you'd need to automatically recognize it which is not easy afaik).