For the older ones, unless there is something that they are over the top involved in (fishing, a sport), I would say stick with a favorite color. If the 14 year old is a boy, a pretty safe bet is blacks and greys or the chocolate brown shades. I would do them in a very linear type pattern. This will still be appropriate as he gets older (my kids took their quilts to college with them). The youngest may have a favorite disney or marvel character you could build around. I would do them in throw size so they are 'portable'.