Use your iPhone's camera to detect and measure your heart rate!
This app detect the color changes when the blood flows into/out of it to measure the heart rate.

Put the tip of your finger on the iPhone's back camera, and your heart rate will be shown.
NOTE: Make sure that your fingertip completely and gently covers the camera lens. Pressing too hard reduces blood flow in your finger and it's harder to get a result. Make sure that your fingers are not cold.

The measurement will be restarted once your finger cover the lens move. So do not move in measuring heart rate.
If the iPhone does not have flash, try to use it in a good lighting environment. The kids could use the thumbnail to cover the lens since the kids' finger is too thin.