You have the ninja family because , though you fed , they're happy hearts were neglected. The ninja family are considered 'bad' characters. You can change them back by feeding them a steamed bun or having them play with the whistle. After doing one of these two things , they should evolve back to the characters they were before they became the Ninja Family.
The Ninja Family happens when you feed your tama, but neglect it's "happy" hearts by not playing with it to keep it happy.
To change it back to normal, feed a snack of steamed bun, or play with the whistle. You can get these items by entering special codes, which can be found at the following link: