Here's a helpful article from Awake21.org on "fasting for kids." A helpful "Fasting Calendar for Kids" is available by clicking here. Prayerfully consider making your 21 day adventure a "family wide project" in pursuing the heart of God.
Fasting for Kids
It is so important to model the principle of fasting to our kids and it’s actually quite simple to introduce different ways they can participate. However, their involvement should come only from a heartfelt desire to, just like our fasting should. The best way to encourage our kids to fast is to keep it fun and simple.
An easy way to explain the benefits of prayer and fasting to kids is to compare it to cleaning out old toys that might no longer work, or that they might have outgrown, to make room for new ones. As we clean out our bodies and make time to connect closer to God and His Word, we make room to receive new gifts from Him.
We do not recommend that children skip meals or drastically reduce their food intake, but a modification of their diet might prove to be an easy place to start. For example, fasting from specific items such as sweets, red meats, fried foods or even introducing a form of the Daniel fast which includes legumes, nuts and whole grains is a great way for children to participate in a safe and healthy way.
Another great way to fast is by reducing some of the time and energy spent watching TV, playing video games, using the computer or phone, and instead choosing to spend time praying, reading their Bible or serving others.
The main thing is for them to recognize the value behind setting aside special times to disconnect from some of the extra-curricular activities and treats they enjoy on a regular basis and learn how to connect to God in a closer way.
Of course, you should always check with your child’s pediatrician before fasting.