In my experience, the CC website is very user friendly for this and you can go ahead and put in the 2 adults and 2 kids and it will give you your options (as few as they are!). I have booked Plaza on the River for 8 of us in two rooms for 2 nights for this summer. PP Westminster Bridge also has 4 person rooms on points. Both would require you to book a Premium Award to guarantee a room for 4, however, Plaza on the River only has one class of room that does not allow 4 people, so you could book a regular award in hopes of getting an upgrade, because any upgrade at all would allow for all 4 of you.