I currently have a standard construction 12`x12` shed that I plan to dismantle leaving the concrete pad in play as a area of support for the shed I am planning on building. I plan on building a 16`x12` or maybe 16` shed with a second floor kids playhouse. When the kids out growthe play area I will use it for storage. If I go with 2x10 joist 12" on ctr would that be strong enough to safely support the second floor? I know 12" on ctr sounds overboard but thats what i used on my 12`x24` multilevel deck using deckblocks and it`s solid as a rock. If this is a crazy Idea let me know,I just thought I could combine the two building I will have to build into one area which would leave the yard more open.

Sorry, I can't recommend a two story structure with dek-block piers. Which you aren't really using here. Which unfortunately I can't give engineering advice for systems other than floating foundation systems.

Sorry for the cruddy answer.