Treat it like it is........a business transaction. 

Your objective is to obtain land on which you can do you thing, without spending the money to buy the underlying base.

Just get the terms in writing, and expect someone else to sniff around looking for land to rent.

It is nice to be "honorable"-------- but you are assuming the kids will function as their father apparently did.........and that is not realistic.  There is nothing in the cards indicating the kids should have "loyalty beyond a check"........they do not live locally, they don't want to be farmers, and they don't want to hassle over the land "out there".

Be thankful that the interloper proved to outsiders, without your intervention, that your price was apparently fair.........but you also just set the base rent for whomever else might want make a run at it.

And, the kids showed you that they are not real sympathetic to the situation.............so, walk carefully, or let the ground go.