Linux is the best option because there are a variety of distributions available for it, so it is the most versatile and can be tailored to the needs of you and your child. 

The best way to teach children how to use Linux is to give them an incentive to use it, and they will be motivated to figure it out on their own. One of the things you could do is install games. You could also have them work on a Raspberry Pi project together with you to help them to learn how to use it. There are a plenty of fun projects online that you could work on.
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