The 850 would be good for the amount of people you will have out fishing, the 650 is still a good manageable size and may work out for you even now. The 850 is going to be a bit heavier, and depending on how you are getting out to your holes, this will be something that you will need to consider.
With the smaller kids out there, you may/could only punch one hole for them, will help to keep their attention longer.
As for the sprays on the hubs - I wouldn't worry about it. Like was mentioned, inside moisture will come from the propane - keep a crack in the door or window, and don't make it tropical inside and you can manage the moisture inside. If it gets slightly wet, set it up once you get home and let it dry out.