Kids are usually quite adept at the butterfly stretch, which works the inner thighs and echoes the criss-cross pose they may sit in anytime they're on the floor! This is also sometimes known as the lotus position—although a true lotus pose means having the feet and ankles resting on top of the thighs, which is very challenging to achieve.
For butterfly: In a seated position, place the soles of the feet together and hold them with the hands. The legs are now forming the butterfly "wings." Elbows can be between the legs (as shown) or resting on the knees. Gently press the knees down to increase the stretch. For a variation that adds a spine stretch, bend forward from the upper back and reach forehead toward feet.