My dd and her friend celebrated their 13th birthdays in WDW. My dd turned 13 there, her friend turned 13 the day after we got home. Their birthdays were noted on all ADRs. We had a breakfast at CMs. Both girls got cupcakes..they thought that was a complete hoot.
I doubt you can expect a restaurant to bring a birthday cupcake for all the kids. I suppose if there is just one addtl child, you might ask, but more that that would be presumptuous. Its a treat for the birthday child, not all the kids at the table..unless you want to pay extra for it.