Children practice writing (pencil writing in the second grade), can they use pen copybooks? What's the difference between a pencil and a pen?

Yes, you can. Both pen and pencil characters belong to "hard pen" calligraphy, and the writing methods are basically the same.

Besides, I can't find a pencil copybook.

Children's calligraphy practice is mainly to learn the "shelf structure" of Chinese characters, and the focus is not on "using a pen".

This is why the ancients practiced calligraphy on the sand with branches.