Several years in the future, despite never getting an identity reveal, Adrien and Marinette do get married and have kids. Still unaware they are both superheroes.
Each night, after putting the children to bed, they each pretend to fall asleep, then sneak away when they think the other is asleep, to meet their superhero partner and patrol the city.
Each believing the other is still home in case something happens. Incase the children need them...
Happily, the very second they both leave the house, a 60 year old version on Bunnyx pops in from the future to look after the children. They don't know about it at first, but after the first time one wakes up and discovers her, the kids quickly begin staying up waiting for her nightly visits.
They know it's supposed to be secret, but toddlers aren't good at secrets. Luckily, Adrien and Marinette are both a bit dense, and just assume "Gramma Bun-bun" is a product the the children's imagination and/or some show they watch.