Starting with the form instructions which I think are always the most important source:
On the form itself the information about your children is in Part 5. Using the section for Child 1 as an example, we filled it out like this:
1.a,b,c. N/A
2. N/A
3. Leave blank since it says (if any)
4,5. Leave blank since they are yes/no questions
6.a. N/A
6.b. Leave blank since they are checkboxes
6.c. N/A
6.d. Leave blank since it is a dropdown
6.e,f,g,h. N/A
Repeat for the remaining fields in that section (Child 2-5).
My feeling is that it is better to fill out the form using N/A where appropriate since 1) the instructions say to do that and 2) it makes it obvious that you intentionally completed each section rather than omitting it.
On the questions above, note the use of N/A rather than “none” since they aren’t asking for a numerical answer (e.g. number of children). If following the instructions as written you’ll find that “none” is rarely needed.