Well, I’m not a parent, so I’m not sure. But I do know there are all types of families on cruises. In fact, cruises are so much fun because they’re a great way to meet people from different cultures and lifestyles (I just got back from a Western Caribbean cruise with RCI). I don’t think anyone would make you feel awkward. Sure, there will be a lot of two-parent families, but there will also be a lot of single-parent families and grandparents with their grandchildren and moms with their daughters… all sorts of scenerios. You definitely won’t stand out.
As for length, if this is your first cruise, I recommend trying a 3 or 4-day first to get a feel of it and make sure you enjoy cruising. Then when you fall in love with it, go back for a 7-day.
I hope you have a fantastic time, whatever you choose to do!