This is a bit last minute since Cinco de Mayo is mañana, but here’s a roundup (btw, is it roundup, round up or round-up?).
You could discuss the Bible story of the talking burro. Even though it’s not Mexican, the burro theme just fits so well! Some other things you might want to talk about with kids and/or students: Mexico, Counting in Spanish, the months in Spanish, the history of Cinco de Mayo, the colors of the Mexican flag.
Do you have any ideas and/or traditions for Cinco de Mayo?