Tails had gotten curious about what would happen if he mixed Sonic and Shadows DNA. So being a curious fox kiddo, he did just that. Creating a hedgehog child. So now, Sonic and Shadow were stuck with a child they didn't know anything about.

Sonic “So what do we do with it?”

Shadow “Feed it and stuff, dumbass.”

Sonic “Well, yeah. That's obvious. But, it's a child Shads.”