It's good that you have a job already, OP. I appreciate that working more than a couple of days a week can tricky with small children, especially if you're mucking in with the farm work on top of that. If you can, put aside a bit each month for a private pension and/or savings accounts... and remind yourself (and your PILs, if it's a livestock farm) that if the female sex didn't have babies, they wouldn't have a farm to run!
Your FIL is unlikely to hand over the farm. Farmers do not retire; they just keep on going forever (I've known one to be discharged from hospital in the morning, and he was then off around his cows an hour later...), and probably wouldn't know what to do with themselves if they did retire. Can you and your DH afford to keep the farm in the event of FIL's death, given the inheritance tax? Unfortunately, it will be difficult to avoid your PILs if you live on the farm.
You and your DH need wills. Even if you're his next of kin, if he dies intestate, it could take a long time to sort out, which would put you in a more precarious position. Is the house in his name, or your PILs? Do you have family that you could go and live with if necessary? If all else fails, if your DH has a lot of good quality land and that passes to you in the event of his death, you could rent it out, as land is a big asset to have, and it would provide a regular income.
I apologise that I can't be more helpful. I don't work on a family farm anymore, so it's different for me, but good luck to you, your DH and DC.