Bowser hadn't taken naps before. Not with a kingdom to run. There were always financial plans to approve and delegates from other kingdoms to meet. He was king, after all, he couldn't let his people down by ignoring his duties.
But now he was more than a king, he was a father. And this was completely new and tiring to him. He'd been raised to go to meetings and talk to people, he'd known what to do if there was a drought before he was even out of diapers. But nobody had taught him how to raise a child.
So when said child was crying and wouldn't stop unless he was being held by him? Well, Bowser obliged. And if that child fell asleep in his arms only to wake up when removed? What could Bowser do but lay down with his infant son on his chest and let him sleep?
The kingdom could wait a few hours for a nap.