Here's a short list of activities that could be fun to try:

Begin by giving the numbers 1 through 9 to 9 students. Here it's good to have a physical number to give them -- a piece of paper with the number written large will work. Ask them to line up in order. Ask 7 whether he or she is even or odd (you don't have to remember their names if they are holding up numbers). Ask 7 about the numbers next to her -- are they even or odd? (5-year olds won't automatically know that even numbers are surrounded by odd numbers. They might not know the meaning of even and odd until you run the activity.)

Further activity: Have only 1-5 stand up in order. Then rearrange them using only transpositions (say "Number 2, switch with Number 5"). Have the students count each transposition. Then have a (well-chosen) student try to put them back in order using only "switches". How many switches did it take? Can 5-year olds discover the sign of a permutation? How about if you record the number of switches and point out even/oddness?

Further activity: Have only 1-5 stand up in order. Have them shake hands in pairs and try to have the others count the handshakes. How many handshakes were there? An even number or an odd number of handshakes? Can 5-year olds discover how the parity of "n choose 2" depends on n?

Further activity: Have the numbers 1-9 stand up in order again. Have them find a partner to add to 10. Then back in line again. Have the even numbers step forward, and the odd numbers step back. Then the even numbers back and the odd numbers forward. Then back in partners to add to 10. Are evens partnered with evens? Odds partnered with odds? You can ask lots of questions and keep the kids moving.

Watch out -- you might have to bring extra numbers and modify groups so that all kids can participate.

Good luck! When in doubt you can ask the 5-year olds why 6 is scared of 7.