From the Super Mario Wiki
Baby Wario is the infant self of Wario. He appears only in Yoshi's Island DS, with other babies. Not much is known about him, except that he appears to be greedy like his adult version. Baby Wario's special move is "magnetism", in which he pulls out a magnet which attracts coins. His magnet is also capable of raising metallic platforms and moving large, metal boxes.
When Baby Wario was originally kidnapped, the Toadies find his tantrums to be very annoying, so they leave him alone in World 3-5. Some time after that, Yoshi, who has been washed away by a large wave and separated from the Stork and other babies, finds him. Baby Wario assisted Yoshi throughout the rest of World 3, but he gets away from him after World 3-8, joining up with a group Bandits in hope of even more riches. Yoshi and the rest of the babies meet up with Baby Wario one last time in Bowser's Castle, having an argument with Baby Bowser. He helps the Yoshis by defeating Bowser on the back of Purple Yoshi. After the castle is destroyed and the team finds the babies, Baby Wario lands on a very large Treasure Chest filled with his treasure. However, Baby Bowser falls and lands in the treasure as well, initiating an argument over it.
At the end of the game, it is revealed that Baby Wario is one of the seven Star Children.
Baby Wario shares many physical qualities with his adult counterpart. They both have a cap and a big, pink nose. Under his cap, brown hair can be seen. Like Baby Mario and Baby Luigi, Baby Wario wears nothing but a diaper. Since he is a baby, he has short, stubby hands and feet.
Baby Wario's cry is the loudest out of all the babies in Yoshi's Island DS. Just like his adult counterpart, he is very greedy. When Yoshi and the babies get to World 5, Baby Wario is seen arguing with Baby Bowser. Unlike the other babies, he talks while arguing. During the final boss battle, Baby Wario rides a Purple Yoshi.
Official Profile and Statistics
Yoshi's Island DS
Instruction Booklet Baby Abilities Information
With his magnet, Baby Wario automatically attracts metal (including coins) around Yoshi. Press Up on or jump to attract things above Yoshi.