If they have questions about the paternity of your child, they will ask for DNA tests. You can send affidavits, but evidence that you were together about 9 months before the birth might be stronger; do you have passport stamps from a visit around those dates? If so, I would include them. Personally, I would not add a comment about "this is the trip our child was conceived", that would be TMI for me, and, if they do have doubts, they will request a DNA test anyway.