Sirius loved his godson just as much as his father did. Even though he was a handful, like all youngsters were at his age, having Harry in his life to care for gave Sirius' life a sense of purpose again. And he could never find himself able to be away from Harry for more than a couple of hours at a time.
One day, Sirius had asked Tonks to babysit him for the afternoon while he went on a brief secret mission. Despite how much fun a babysitter Tonks always was, Sirius came back early. Tonks' babysitting job was easy.