"Guessing carefully" is closer to the truth than anyone likes, and it's worth saying so plainly. Here's the shape of it: topical tretinoin is absorbed through skin in very small amounts, and the theoretical amount reaching milk from a pea-sized nightly facial dose is considered negligible by many clinicians — which is why some are comfortable green-lighting topical retinoids during lactation. Others advise waiting until weaning, on the logic that the vitamin-A pathway is the one with known oral-exposure risk and the direct lactation data is scant (the same evidence review that guides pregnancy advice covers how thin this literature is: https://pmc.ncbi.nlm.nih.gov/articles/PMC3114665/). Both positions are defensible readings of limited data; neither is guessing recklessly. That's why your answers conflict.
So the honest move is: ask your own OB or pediatrician, tell them it's facial topical use at cosmetic doses, and follow their comfort level — this decision is cheap to defer and cheap to make with them. If they clear it, common-sense hygiene applies: apply at night, wash hands after, and avoid any chance of the baby's skin contacting treated areas (cheek-to-cheek snuggles land exactly where you apply it).
If you'd rather not reopen the question while nursing, you're not without options: azelaic acid is the standout — real evidence for breakouts, pigment, and redness, and broadly considered compatible with lactation — plus niacinamide and consistent sunscreen. Many people find that trio holds the line well enough that tretinoin waits comfortably until weaning. Your skin noticed the absence; it will also notice the return.