Go for a long period of time just planting one of each crop. If you don't
have enough money to begin with, try selling the children. You only get $10
per child, and only if they have full health, so best to do it closer to the
start of the game. Also, if you hav a single adult running the farm, try 
marrying and them making that spouse leave the farm. You get $20 for the 
healthy spouse to leave the farm, but the amount you get when they join 
the family varies. Can only do once per turn and there will not always be
a suitable spouse.