Children, including one with a painted face, pet tiny turtles that are crawling in the green grass.
Turtles are crawling in the green grass.
"Tiny turtles are crawling in the green grass" is a rephrasing of "Turtles that are crawling in the green grass."
If people are petting turtles in the grass, the turtles must be crawling in the grass.
The children are petting turtles which are crawling in the green grass.