On this particular subject, you will receive different opinions. Keep in mind that we are not attorneys. If you go by the N400 instructions, on page 9 it says:
Item Number 1. Indicate your total number of children. Count all of your children, regardless of whether they are alive, missing, deceased; born in other countries or in the United States; under 18 years of age or over 18 years of age; married or unmarried; living, current stepchildren.
I was in a sort of similar situation, with some differences. But I also have a stepson, though he's a minor. I did not think I had to list him on my I-485, but when I started filling out my 751 and had to answer about children something rang a bell on me. So I consulted two or three different attorneys, one of which is a top notch one, and their answers agreed that "children" is a broad term for USCIS and stepchildren are your children. So, knowing that, I did include him in my 751 and plan on doing so this week on my N400.
I know you did not include him before, but from my conversation with one of the attorneys, since your and my stepsons are both USCs, this should be a non- issue, because you are not granting him any immigration benefit.
Actually one of the attorneys I asked is Jim Hacking. I was on one (or several!) of his live shows. I just can't find the clip.
You might want to consult with some attorneys, too. I just personally think it is better to be honest about a mistake you've made, than to be perceived as concealing information. My two cents.