In the begining of Back to the Future Part II, when Doc arrives in 1985 to get Marty and Jennifer back to 2015, he apparently knows about the car accident that ruined Marty's life in the future. This guess can be deduced from the Part III where Doc gives sort of hint to Marty about that accident. Obviously Doc only could find out this event during the first trip in 2015, before the mentioned moment.
Why does Doc care about the trouble with Marty's kids in the distant future and ignore the key event in Marty's life which should be happened literally the next day? The event most probably caused all further troubles with Marty's family including the above.
No need to send him to the future to fix it. The accident will happen just the day after. So all that's needed is warn Marty of the accident. Then go to 2015 to track the effects. And most likely there will be no need to save Marty's kids.