Other than Victoria & Albert - every restaurant at Disney is child friendly.
Your 4 y.o. is the 'problem' with staying for fireworks at CA Grill. But Artist Point is no better - since any 4 y.o. will rarely do well sitting through a nice meal - especially if you've been in the Parks earlier in the day.
I remind my child that when she was young - she ruined each and every sit-down meal we ever attempted to enjoy at Disney.
Then we learned about Camp Dolphin (and we stay at the Dolphin & Contemporary).
Disney is closing their 'Camps', but Camp Dolphin is a 'free' service if you buy an entree at any of the Signatures at Swan & Dolphin. Some restrictions apply.....
https://www.swandolphin.com/kids/campdolphin.html
But if you really want to go to CA Grill or Artist Point - I think CA Grill is the better option. Your two older children may be able to help create diversions for the young one by taking short walks.
Which offers another option - Narcoossee's - they also have an outside deck for creating diversions....and fireworks view.
Fourth choice, maybe something to keep the young one engaged, and is fun for the entire family - Hoop Dee Doo Revue. The food is meh, but a fun time can be had by all who attend. It's certainly not a Signature experience, but 4 y.o. kid behavior is a guessing game after a day of Park stimulus.
Happy Birthday, Everyone!