For children you can do this to count how many times they can sit down and get up without breathing and they will have a good time. Don’t rush because moving faster takes more power than it is saving. Move at a comfortable speed and control.
Many people panic when they are out of breath. to learn the vast resources and the key to them work in this way among others: Take a simple drill like walking or push ups and do a few steps or push ups without breathing. Count how many breaths it takes you to recover and calm down and notice the time you spend recovering will be smaller than if you do not pay attention (awareness brings relaxation and better performance). Now do a few more and this time listen to your heart beating in the area you are working from legs to arms and count the heart beats. This too spreads your awareness and allows for more relaxation. Think about your own levers of control: your heart rate, temperature and blood pressure and notice how they feel as you do the simple drills. Now continue to count (it keeps the mind working) the breaths and the heart beats as you progress and in each time you do this work with the breath to recover through less breaths. Pay less attention to the actual time you need because when your breath is calm the flow of time slows down. Breath holding can also be done underwater though you should have a friend around in case you will need help.