Haven't read all posts, so apologies if someone has said similar.
IMO as someone who has been cheated on, I would possibly suggest OP doesn't tell, if it was a one-off.
The risk to her family and marriage, effect on DC is so great, would it be worth it?
OP will have to live with the guilt for rest of life but why punish the rest of the family, if she is remorseful and certain it wouldn't happen again.
OP needs to look deep into herself and find out why it happened, and change, if she wishes to remain with DP and loves him.
However, if it then went on for longer, other one night stands or developed into an affair, then I feel that would be different.
The lies and deceit that affairs bring are most devastating to the one cheated upon, I know from experience, and yes you are made a fool of, I wish I had known sooner, but I didn't find out for several years.
Then OP should tell DP. If only out of respect. My ex h showed no respect for me or my DC.
I could have forgiven a one night stand, but not the years of living a lie.
Hope this doesn't sound contradictory.