Questions you should be asking:
1) will you want a bathroom? do you plan to show in it?
2) do you see yourself cooking inside or outside most of the time? How out about doing dishes?
3) what are the age and gender of your grandchildren? a king sized bed will easily hold 3 kids, especially if they are small (thought they will grow). One way to look at a king sized bunk end is as a 3 person tent. They are about the same size. A fourth kid in the same space will be a bit tight, esp as they grow older, but that is ok, because they all have dinettes that can convert to a bed, and is actually too small for 6 people anyway, so mostly you will be eating outside.
4) how much can you tow?