Google "printable yoga cards for kids" and you can get some resources... here's one that's really cute: http://speakfromtheheartup.blogspot.com/2012/05/yoga-for-kids-free-printables.html
If you have a printer you can print & laminate them. Or you could save them as separate images on your computer and make them into a slideshow.
DS & I sometimes attend free yoga classes in our area and that has given us tons of ideas -- often we just replicate class at home. And we just come up with different animal poses, making them up as we go along, and it's lots of fun. Sometimes we play kids' music (especially animal songs) for inspiration. You can buy kids' yoga CDs but we just use whatever music we already have. If you do buy a CD, I recommend Come Play Kids by Karma Kids. Also, don't limit yourself to yoga books, you can take regular books and do yoga for each page (like Brown Bear Brown Bear -- do a bear pose, a horse pose, etc.)
Also try breathing activities -- blowing pompoms with a straw, laying with a toy on your belly and moving it up & down with your breathing, blowing feathers, etc.
Kid yoga is a lot more fast-paced than adult yoga!