While formal nights are not what they used to be, the dress code asks for a dark suit and tie for men and a cocktail dress for ladies. If you want to pass on dressing for dinner, there will be casual options that you can select. Jeans and a polo would not be acceptable in the dining room on a formal night, but I suspect that you will see some folks dressed that way.
Well it all depends on if it is a formal night or casual night or semi-casual night. For the formal night i would recomend a dress for a lady and a nice suit or even a tux for man. For the children the same. Now for semi-casual your sons outfit would be fine and for a lady a nice spring dress and almost like a golf outfit would be good for a man. Casual night is the night where you can wear the outfit you wore out on the excursion port. But if you feel to casual just put on a fresh t-shirt and some jeans. I think sandles would be a bit too casual.
For formal nights in the main dining room, one should wear something that would be suitable for a traditional wedding, I think. If not a tux or dark suit for a man, definitely a sharp jacket and co-ordinated pair of slacks, crisp shirt and tie, dress shoes. For a lady, some outfit of elegance and a sense of dressiness is appropriate. The same goes for children.
If you prefer to be more casual, dinner in the Windjammer would be perfect.
If your son is, say, 2 years old, it might be appropriate for him to wear something less than a suit. I don't know anything about RCI but I know that on other lines, very little kids can wear pretty much anything and be okay.