I recently came across "Sitting Still Like a Frog" meditation for kids in Eline Snel's book Sitting Still Like a Frog: Mindfulness Exercises for Kids.
The author talks about how the frog is capable of enormous leaps, but it can also sit very still. Even though it is aware of everything that happens around it, the frog does not react right away.
Instead the frog sits still and breathes, preserving its energy rather than getting caught up with all the ideas popping into its head. In her book, Eline Snel encourages children to pay attention to their breath, and be still and quiet, just like the frog.
After taking a few moments to simply breathe and letting go of all the chatter going on in their minds, have kids jump up in the frog pose and say "R-r-r-r-i-b-i-t", taking a giant leap just like the frog!
This exercise can not only help kids with their concentration, but can also help them to be less impulsive by teaching them how to not immediately act on what they are thinking or feeling. Instead they learn how to pause for a moment and just breathe.