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.
I do not understand that what is the need of teaching a 7 year old about bitcoins. By the time he will reach 20-25 and start earning, bitcoin might have disappeared.