Your pre-kindergarten students may not be able to read much yet, but you can help them develop a Spanish vocabulary. Make up a bunch of index cards that have the Spanish vocabulary words on them. You can either go through these as flash cards or attach them to the objects in the room that they represent. You can place a card that reads "la ventana" on the window or another one that reads "la puerta" on the door. You will want to use mostly objects instead of trying to teach your pre-kindergarten students about verbs and conjugation. Keep the vocabulary words simple. You can make up a contest to go throughout the month and at the end of the month give the student who learned the most new Spanish words a prize.
Teach your class about the parties Hispanic children their age might have in other countries. You can even celebrate the end of Hispanic Heritage Month by having a party. At the party play some of the games children in those countries might play and end the party with a piñata, which is a typical party game for Hispanic children of all ages. Play music from those countries and try to have some of the instruments for the children to play themselves. You can also create good luck eggs. You will have to plan ahead because you need to hollow out one egg shell for each student in your class. You want to make the hole in the bottom of the egg about the diameter of a pencil. The kids can then fill the eggs with confetti and cover the eggs with tissue paper or color it with marker. When the eggs dry take them outside and allow the students to crack them gently over other students' heads for good luck.
Sing songs with your pre-kindergarten students. Sing songs that they know the English words and the tune to, but sing them in Spanish. There is a wide variety of songs that you could sing with your students, such as "Twinkle, Twinkle, Little Star," "The Alphabet," "Row, Row, Row Your Boat," etc. If possible, try to find recordings of these songs on CDs to help the children learn the new words faster since they cannot read the words if you write them out on poster board. You can first sing the English version of the song and then the Spanish version right afterward, as if it is a second stanza or chorus. You can also create motions to go with the words of the song to help the children remember a little bit more.