Smocks can simply be an old (bit larger) shirt that can be pulled over the head. If using a button down, please trim sleeves.
As a note, while I do have extra smocks for special situations, I do not keep a supply for each child. I believe it is better to have one that fits them properly and that is not shared by others so that we can keep as healthy as possible! In addition, it saves precious art time when they come in ready to create.