There are always days when it seems like the last thing the kids in my yoga class want to do is.... yoga! So this is when it takes patience and creativity to get the children's attention. These are some of my tricks that I use depending on what feels right in the moment:

Musical instrument- A Tibetan bell, drum, gong, finger cymbals, or any other musical instrument. Ring it, beat it, and play it to get their attention. Saves your vocal chords and works like a charm...
Whispering- Instead of yelling, try whispering. Sometimes talking very softly will have a quieting effect on the chitter-chatter, especially if you make it sound like you are trying to tell them a VERY important secret!
Singing- I love to sing, and often I break out into spontaneous song instead of yelling. I sing things like: "Everyone back to your mat"; "I'm waiting for you to be quiet"... Operatic style. It always seems to get their attention.
Striking a New Pose- They might not be paying attention because they are bored... Sometimes I go into a new, challenging yoga pose to get their attention... Try Crow, King Dancer, or a spontaneous Handstand against the wall. It tends to peak interest...
New Music- Try changing the energy level of the music. Put on some upbeat groovy music for yoga flows, Sun Salutations, or free dancing.

I switch up these various tools to keep my students' attention (and my own sanity). Have fun and don't forget to keep your sense of humour!