They look really good for first season seedlings. There are no set rules or expectations for all tree seedlings. Many don't really take off for a year or two and others go nuts the first year. Even in a single batch of seeds from one plant this can happen.
What kind of germination percentage did you get? Did you warm stratify and then cold stratify?
Just put them outside in January. I think the percentage was less than a third, as I recall. Might have been better if I had planted them deeper. I think directedly into coarse soil (my first trial doing this) might have to be deeper to prevent disturbance and drying out.