First off, you have a one year old. Your relationship has probably never been through so much stress. It seems, in my limited experience, pretty normal for things to get rough at this stage. I seriously considered leaving my DP around this stage too, convinced the relationship was doomed.
But we got through it, and are stronger than ever. Surviving having a baby (we're now on no. three), I think we can survive anything together.
Secondly, everyone has a different idea of how bad this kiss would be. There's no right answer. But what's for sure is that your DP is probably primed to take it badly. At this stage, you could have a world ending argument over the washing up. If you do decide to tell him, maybe don't blurt it out over breakfast? Maybe do it in the safe(er) space of couple's counciling in coordination with the counsellor. Maybe wait six months or a year. There's more nuance here than tell him/don't tell him (although obviously the longer you leave it, the more he may feel lied to).
FWIW, I wouldn't take it badly, if DP confessed today to a kiss, back in our first year of parenthood. I'd see it around the way I'd react if they drank 10 pints, came home, and were sick in the front garden. I'd think they were a bloody idiot. But then, we're the both of us bloody idiots sometimes, but we're also deeply in love, after all we've been though together, and we're got eachother's backs. At the end of the day, it would illicit sympathy, not resentment.
Good luck OP, get yourselves into couples councilling!