Baby Kid Buu is a character created by Supreme Gogeta.
Baby Kid Buu was born the day Baby Vegeta and Goku started to battle. He was created/born because he saw Majuub still had a little evil in him, so he absorbed him and that created Baby Kid Buu.
He was killed by Super Sayian 4 Gogeta with a Big Bang Kamehameha, right after Gogeta talked to Majuub it brought him back to normal and he was able to become free from Baby's control.
He is a bit stronger than Super Saiyan Gogeta, but is no match for a Super Sayian 3 Gogeta.