How many more are you? We showed up with two extra at Chef Mickey's (we were a party of 11 instead of a party of 9). They accommodated our extra guests fine. They did the same at CRT lunch.
If you are doubling the size of the party, perhaps that's an issue, but it may not be a problem.
Is Chef's Mickey's sold out? If not, can you get a separate reservation for the others? Party of four or whatever. If you can, grab it, then show up together and they'll probably be able to seat you together.
In any event, I'd keep what you have but book a later back-up, which you can cancel if they are able to accommodate you at CM's.
Ideas for a back-up for 4 year old boys' birthday:
Garden Grill at Epcot is a Mickey character dinner, but requires Epcot admission, which may be an issue.
Whispering Canyon Cafe at Wilderness Lodge can be entertaining for kids, though not everyone likes that brand of humor.
Boma at Animal Kingdom Lodge is a nice buffet (no characters).
Cinderella's Royal Table for dinner. No princesses plus boys often find it fun to eat in the Castle. The Fairy Godmother is there to help celebrate birthdays.
Coral Reef (Epcot) -- our boys loved it at that age (tank full of sharks, turtles, etc.)
'Ohana at the Poly -- we don't care for the food, but many love it. There are coconut races and other kids' activities. Some table have fireworks views.