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.
That's right, the best way to explain is we should talk to them, i think they could understand well. Children must interesting about this because of about money related. We need them first understand about bitcoin before we teach. Give more patience, to teach them so that they have more ideas and knowledge about bitcoin. Children is not so hard if we explained it well. Just keep telling and have patience to explaining and focus to teach children to them always so that in their younger age they will know a lot.