The n-400 instructions state:
A. Write the total number of sons and daughters you have had. Count all of your children, regardless of whether they are:
1. Alive, missing, or dead;
2. Born in other countries or in the United States;
3. Under 18 years old or adults;
4. Married or unmarried;
5. Living with you or elsewhere;
6. Stepsons or stepdaughters or legally adopted; or
7. Born when you were not married
This means all children, including stepson(s)/stepdaughter(s) from any previous marriage(s), regardless of it you have contact with them or not.