Explain them why blockchain and why centralization is the problem and how blockchain works.
Introduce them to bitcoin ,and it's purpose.They will eventually learn about altcoins too later
But always know the difference where decentralization can be applied well and where it can't. Developers should not always apply decentralization just for the purpose of it. I believe good examples of "where decentralization is not needed" are shown in most Ethereum dapps.