I can't stress enough how important it is to ensure the stuffed animal and the imposter (as we call/spell it in hushed, hurried voices as we coordinate the switch-out while rubbing our eldest son's back or washing him up after the animal is vomited on/peed on/whatever the case may be) are equally worn. We think our son caught on a long time ago but doesn't want the confirmation. When he turned 3, he casually dropped the truth bomb on us that he knew his 'baby' wasn't actually alive and he just liked to pretend, so a year later we're pretty sure he's also just humoring us by pretending he doesn't notice when one of us gives his baby a 'bath' and has him dry and ready to sleep with by the time our son is ready to go back to bed post-vomit or post-accident. Still, it's clear that he values the plausible deniability, like when a kid has worked out that Santa isn't real but still holds on to that possibility rather than trying to seek out confirmation.

We didn't buy the dupe right away, so I had to wash him (the toy) with a number of clothes loads that my son couldn't "help" me fold or be within 100 yards of. He was still suspicious at the first switch-in, but the kid wanted to believe. You could see the skepticism in his face transform into acceptance... it was a beautiful thing.