Welcome to the extraorindary, illogical (to adults) world of children / teenagers.
Firstly all cereals with the exception of shredded wheat and natural oats are full of salt and sugar but that’s neither here nor there.
Children as well as adults go through phases and making too much of an issue will create more of a problem. Yes, Dinner time is dinner time and if the 12 year old doesn’t want what is served ask him/her what they would prefer but do try to give it at the alloted time of your dinner. Try asking why they don’t want what your wife has made. Maybe they are not yet hungry. Maybe dinner is too early. Ask what the problem is. 12 years old is old enough to sit and talk with the child.
Treat the 12 year old as a budding teenager and you will find that you are quite capable of getting an acceptable response