Preschoolers . . . so 3- to 4-year-olds? The best thing is to keep them engaged in doing something (don't sit and talk to them for too long), and change activities every 10 minutes.
Their involvement can be very simple. Something that we think is silly, they will find fun and entertaining. For example, singing songs and doing actions. Kids love to sing - I would start there. Move your hands (have them do it with you) up and down as you sing, indicating what note you are on. I've even seen some people teach with hand signs for do, re, mi, etc. as they move up and down. Or, have them each hold a picture of what they're singing about...like stars for twinkle (they can hold them up high for "up above the world so high").
March around the room and have them follow you in a line and copy the rhythmic steps you make (along to music and/or singing).
Let them take turns standing in front of the group and holding up signs - one that says "f" for forte and one that says "p" for piano. They have one in each hand and get to hold up one at a time and switch it whenever they want. The rest of the kids have to sing through songs and follow the signs, by singing loud or soft. Of course, you will have to explain to them what the f and p mean...then model what to do. With any of these games, expect it to take a few tries or more before they catch on.
Some kids this age do not pay attention long enough to make much eye contact, and it may seem like they are not paying attention. Just keep them busy and keep encouraging....you'll be amazed at how much they are soaking up when it seems they are just being wiggly. Oh, have them sit in a designated spot....a blanket or carpet squares or something.
Do you have any bells, tambourines, triangles, xylophones, sticks that they can play?
After a number of lessons, if you want to teach them notes...start with the staff and talk about the lines and spaces for a while first. Instead of using your hands in the air to move up and down with your voice, show how a note can move up and down on the lines and spaces. Things like that. Then introduce letters. I made puzzles of piano keys and matching letters for my 3-year-old students to put together at each lesson.
In addition to showing them pictures of notes, let them draw. They can make circles and lines no problem. Again, keep them busy doing something!
Talk to them in a cheesy, excited voice. You will feel silly, but they will love it. I hope this gives you some ideas to start with. If all else fails, have a bottle of bubbles ready!

Good luck!