Butterfly pose is a favorite for children.  Once they motor plan the task of scooping legs into their arms, they establish balance.  You can take it one step further by asking the child to clap their feet together and bark like a seal.

Yoga Journal recently shared a suggestion to lie on a swimming pool noodle to encourage chest expansion.  As I am always interested in new ways to encourage heart opening while stimulating potential for greater respiration, I tried it!  Love, love, love this restorative pose as it energetically opens the heart and physically provides extension and expansion for deeper breathing.
While your child is sitting on the mat, place the noodle in back of them and have them lie down and align their back and head on the noodle.  Be sure this is comfortable for the child as you may wish to slightly slide the noodle up away from the sacrum
Best bit of news is that Tundra is selling small noodles that are perfect for little backs.  Three minutes seems to do just fine.  Please let me know if you receive extra hugs from this heart opening pose.