One good hypothesis to formulate for a child that age would be that a person's heart rate is influenced by their physical activity. You could have them measure their pulse (as beats per minute) after four different activities. The first time would be right after waking up in the morning. The second would be after getting ready for the day and eating breakfast. The third would be after taking a regular walk around the yard. The fourth would be after a brisk run around the block. After each activity, measure your heat rate in beats per minute by taking your pulse either at your neck or wrist. Record your observations on paper. You can then rank the four different heart rates in numerical order and see if the hypothesis is true.