I think we can start explain bitcoin by tell them that bitcoin is like other international currencies dollar, euro and many else. They will understand and you explain more about bitcoin.
and what is to be done if they asking "why we couldn`t mine dollars like bitcoins?"
Because they are not exactly the same thing but they have similarities in their use. Both of them can be used to buy something that has a price. We can't mine dollars because dollars are printed out, not mined. Definitely you can print out more dollars if you're the government. Otherwise, you're gonna have to work to get dollar just like everyone else. The same thing applies to bitcoin, if you want more bitcoins, then get a work and be paid with bitcoins.