First, order picture keychains. They're absurdly cheap -- I got this set of 12 for only $9. Then, all you need are ink stampers (or paint), white card stock, and a thin Sharpie.
On one piece of paper, get a thumbprint of the child. I found the easiest way to do this is to ink the entire hand and then bring the paper to the thumb only. This is especially true for young children. Use the Sharpie to write the child's name and date.
Teachers love puns! On another paper I wrote, "Mothers forever leave fingerprints on their children's <3s"
I pried open the keychain with the edge of my scissors and used the paper inside to trace around the thumbprint and wording. I cut them out, placed them in the keychain, and snapped it shut.
The best part is that I have exactly 12 students total, so I didn't have to buy any extra keychain sets. I'm looking forward to wrapping these up and giving them to my kiddo's moms!