If it is the first time, the kids may be nervous or excited. I would recommend starting really slowly. Actions are great, because kids can just imitate you. Start out by saying "jump" have the kids jump, clap hands, sit down, stand up, etc. Don't worry if they don't speak at first. After doing this for awhile, I recommend going into something very simple, like numbers. Something they are already maybe a bit familiar with. I have a free numbers song download you can use: Free Numbers Song for Kids MP3 download: Let's Count 1-10,1,2,3,4,5,Jump!
The idea above with head shoulders is great, first slowly teach the vocab, and then have the kids follow you along. I have a free download of this song too: Head, Shoulders, Knees and Toes-Free mp3 Download, free kids songs
After you have warmed up and relaxed the kids, sit them down and read a book to them. I have free books you can use here: Free printable kids, childrens English picture books
Start off really slow and easy with the kids, and they will respond well. If you want to try "what's your name?" as mentioned above, I also have a free download for that: Kids English song What's Your Name?
Hope this is a bit helpful! good luck!! -Matt