They proposed higher than market rent, but want to keep 2 people from thee organization volunteers in one room in future.We checked the website of the non-profit organization, their program include several categories, including helping international children to school, housing for the homeless, etc.

Know that children are rough on properties versus older adults with no kids.  

I have a husband and two small children.

We were planning $600/mo mortgage payment, $100/mo insurance & $100/mo taxes - balloon payment in 5 yrs.We are providing $5k to pay back taxes ($4k), back hoa ($125), pump septic & grease trap clean out ($225), survey ($600)  & our children are paying closing costs ($1500).Deal stalled as owner backed out on seller finance.Suggestions?

I would not sign a lease and buy a bunch of furniture to potentially make $350/month. 

Couldn't agree more with replacement costs... a lot of BnB-ers trash your furniture.

Anyhoo, my long term goal is to build a rental “empire” to pass down to my children and so on. 

He can also leave any furniture behind and we will clean up.2 – Flexibility to close/move in on date at his convenience.What other suggestions should I make to make my offer better?

But you may be able to do option A for $10k or less in setup & furniture, esp if you buy the place fully furnished.

Let me break it down in simple terms.In Massachusetts, if your property was built before 1978, you cannot rent it to families with children under six unless it complies with the Massachusetts Lead Law.