For the older child, bury three types of toys- up to 15 total items. For example you could bury seven lizards, three dinosaurs, and five cars. After graphing, ask questions such as "Which toy do we have the most of? Least? How many more lizards did we find than cars?"
Younger children may have a hard time focusing on the graph while still in the sandbox. Take the treasures inside and create your graph. Count and talk with your child about how many of each toy you discovered.