These were simply tempera (poster) paints I got from the local discount store. I had them squeeze small dollops of the paint in various positions. Next, they simply took a straw, and without touching the paint with the straw, blew softly or forcefully, whatever met their fancy.
The end result was pretty fantastic, right? I have a feeling DD8 (first picture below) didn't just blow, but maybe used her finger or the end of the straw? ;)