I've noticed that some of my tree peony seedlings look like they already haven't made any increase in their leaf size in over a week, and it's only the beginning of April.

Is there any chance that they will produce more leaves later in the summer or are these guys pretty much done for the year?

I've never seen secondary growth from tp seedlings, but I've also never had them completely grown before April before. I know that fruit tree seedlings typically stop growing, and then start up again to make more growth on the same year. I'm guessing that might be possible with these guys. Here's a picture of how most of them look now.

Here is a link that might be useful: Fully grown TP 1st year seedling with 2 leaves