During Chapter 6, directly after getting off the boat, you'll notice a large turtle in the middle of the water. Go to the right side instead of following the female, and you will notice a total of 3 baby turtles in this area. You will need to deliver all 3 of these turtles to the slide where they will drop into the water and be reunited with their mother. Just use the brothers to drag them to the appropriate spot, and remember to use big brother to boost little brother up the ledge to get to the highest one.
This achievement can be seen in the video guide at 9:51