Ask each member of the family to itemize their possessions on paper. It seems like a dreadful task, but it will be worth the effort in the end. Item by item count isn't necessary, but you need to know whether you'll be storing more shorts than long pants, or skirts and blouses with only a few dresses, before you begin making decisions. And then, of course, you must account for shoes and accessories. The above chore will help determine how much full length hanging space is necessary, compared to double hanging space.
Corners are good for shoe storage.
Shelves over drawers are great for folded sweaters and folded pullovers. Hide the underwear in the drawers under the shelves. For growing children, the best bet is to accommodate their closets as if they were adults. Since they seem to outgrow their clothes every three months, it is unwise to make their storage pint-sized. In the bathroom, keep safety considerations first.