It sort of depends if you are willing to take the risk or not.
While Cedar fair parks have a history of being bad with the rain, On the positive side, the lower attendance that results from an overcast means the lines are short enough to get close to the same amount of rides you could get on a busy day, in a much shorter time period, Which is important in this case because that short time period is your window of opportunity where the management isn't closing rides because of rain.
If I were you, I would urge your family to stay at Cedar Point's indoor water park (Castaway Bay), that way if the park is running terribly; your family will still have a good time and wouldn’t be reluctant to make another 9 hour drive in the future.