Your students can copy the sample cover letter or write their own, but seeing the sample will provide them with some guidance. The cover letter could be as simple as a statement attesting to the fact that the work is the child's original work and inquiring if the poem or story might be of interest to the people who read the magazine. The teacher could provide an information sheet for each child to fill out. Place the poem or story on top of the information sheet and the cover letter on top of that. It would be nice if all three were typed.
Publishing companies receive many submissions throughout the year.
A sloppy submission may not get the same attention as one that shows effort to put together gets. Appearances do count. When material is submitted makes a difference. I believe the earlier in the school year material is submitted to a magazine, the greater the chance it will be accepted for publication. My reasoning is that by the time spring comes, children from many schools have an arsenal of material they can and do submit to magazines, so the later in the school year it is, the greater is the competition for a poem or story to be accepted by a magazine.