Child Yokai

In Japanese folklore there are a number of yokai that appear as children. These in tofu kozo, amefuri kozo and zashiki warashi.
Tofu kozo

In Japanese folklore, tofu kozo is a yokai appearing as a young boy, wandering around offering tofu on a plate. Some say he's harmless but others believe that the tofu he offers is poisonous and will kill you. It's also said he gets bullied by the other yokai.

Tofu-kozo from the Kyoka Hyaku Monogatari by Masasumi Ryuukansaijin.

Amefuri kozo

Amefuri kozo are yokai that look like a young boys. They wear a child’s kimono and wooden clogs and are responsible for causing rainfall. But beware as they enjoy stealing umbrellas to wear as hats, leaving their victims to get soaked in the rain.
Amefuri Kozo - Toriyama Sekien
Zashiki warashi

In Japan, zashiki warashi are mischief-loving house spirits appearing as young children. Phantom noises in the night or strange children's footprints can be a sign they are haunting a home. They're considered lucky, protective and can bring fortune to a household.
A variant of the zashiki warashi is the makuragaeshi, see separate post here

Another well-known child yokai is hitotsume kozu.