Hi! I have used Orbitz many times without issue. You need to go to the Orbitz site, put in your dates and only put 2 people in the room. Use the code review15 and price hotels. The price that shows on the main page is before the discount, so go ahead and choose a room. Price will be on the right side in the box. There is nothing to really add, if Disney offers a discount it will show up in the price on Orbitz. The code is to take another 15% off. After you book, wait a day or two and call Orbitz (or they have a live chat which works too). Just tell them you need to add children to you reservation. Most employees are familiar with this and will put you on hold while they do it. If they say you need to cancel and rebook with more people, just say thanks and hang up. Call back. I have only had 1 person in probably 6 times of calling who did not understand.
You can link to MDE right away, even with only the adults on the reservation. Print out the confirmation after you book, the number you need to link in MDE is the “hotel confirmation number”, not the Orbitz locator number. You can link the reservation, and then when you call the next day and add the kids, you can actually check MDE while they are calling, refresh once they come back and say the kids are added, and your children will magically appear in MDE (you then must link to their names). Make sure you change the address on your magic bands as they default to a Chicago address. It really is very easy once you know how to do it.
You cannot add dining to an Orbitz reservation. In fact, if you call Disney they will say they can’t help you, changes must be made through your TA (which is Orbitz). You must also pay in full with Orbitz. They do have great cancellation policies, and if you cancel, you will have your money refunded to your card in just a few days (much faster than Disney refunds).
You do get all the normal benefits of staying on property. There is no difference at all, you will just pay less money . Hope this helps!