Hi,
If it's possible I'd recommend trying to see if you can get early exposure to child psych in 2nd year, mainly for your sake. Unless you're 100% sure you want to do child psych, it would be very helpful in your decision process. It would also help in the interview process to have a more informed decision. However, if you can't make that happen, I don't think it should necessarily prevent you from applying in 3rd year. Other then a few exceptions, you shouldn't have any trouble matching into a child fellowship either way. I wouldn't hold off applying to the fellowship until your a 4th year unless you need the time to decide it's right for you. Keep in mind that child psych is not for everyone. I personally enjoy it much more then working with adults (other then schizophrenics...one adult group I can work with), but working with the parents can be very difficult.