If you are going to teach it with the child who is 7 year old, you are going to use the word that can easily understand or comprehend them, but it really depends on the child if the child is not interested they will not easily understand it coz we all know that age is not the age that a child will give time to understand it. There is a child who are very curious but only few.
Even if it is hard to teach like a 7 year old, But if you see that the child is interested to it you can easily teach it. The easy way to explain in 7 year old child, I will tell to him/her that bitcoin is a digital cryptocurrencies and it helps every people to have an source of income.Also I will tell to him/her to read some threads to them to be able to know more about bitcoin.