Hi OP - I book Orbitz frequently, for Disney resorts. Always book 2 adults, and then call to add our 2 kids. (Never do a child initially; they have problems sometimes adding an adult - even though you will not be over 2 adults total.) Never a problem. If you get an Orbitz person who says you owe extra for the 3rd/4th person, either explain Disney doesn't charge for 2 kids, or just hang up and call again. They are so used to this now, they might say they'll call you back after they get Disney on the phone and do the change (i.e., you must call Orbitz - Disney won't talk to you at all about an Orbitz booking - then Orbitz has to call Disney. Sometimes Orbitz puts you on hold while they call, but lately they call me back. If they don't call back within 6 hours, just call Orbitz again.)
Also, despite sometimes seeing the $25 cancel charge on the booking confirm earlier this year on some of my Orbitz onsite bookings (but not anywhere before I booked/paid), I have canceled several (I change resorts alot!) and have never been charged anything - got full refunds every time. YMMV.