Your children will find stability in solid parents, not a house. Also – how do you expect to maintain stability with such a landlord ? Lastly – do you really think this idiot who is upside down is going to honor his obligation to sell you the house? And you have to go through a short sale process? What a mess.
Seeking stability around a flake is just not wise.
Find stability elsewhere.
Get out, get away. Break the lease. If he says he’ll sue, tell him you’ll call the bank and tell them he has been cashing rent checks.
One other thing – a little instability is good for the kids. It teaches them to be flexible. Likely, they will deal with the instability better than their parents.