Trust, transfer on death?

Complicated questions about how to plan inheritance of a house after parent’s death. Will consult a real estate lawyer, but just wonder what options to consider.

The house has been in the family since it was built in the 1950’s.

Goal is for the mortgage to be paid off now by child one and upon parents death child two would own the home and live there.

120,000 is owed on the mortgage and the house value is around 300,000. Parent wants child two to inherit the house, but pay back the amount that was needed to pay the mortgage off now to child one.

No idea how to work this out legally.