In Chapter 6
After you got off the boat you'll see a big turtle. On your right, side you can follow a path to an area with three baby turtles, that are a little spread out. The little brother can push the turtle at the bottom of the area, down the hill and into the water. This will reunite it with the momma turtle. The older brother is needed to carry the remaining 2 turtles up the small hill to get to the slide.
I found a YouTube video for this achievement. I didn't make it.