Mention of mini-blockchains in IBM's ADEPT white paper:

“Multiple efforts like sidechains, treechains, and mini-blockchains are ongoing to address this problem. While each approach has its merits and demerits we are yet to see consensus on a common approach across the board. A blockchain to cater to hundreds of billions of devices needs to be scalable.”

http://www.coindesk.com/ibm-reveals-proof-concept-blockchain-powered-internet-things/