Onigo Musume a.k.a. Onigokko are an elementary school group created by AOS / Atena Music that first formed in February 9, 2012. They were being promoted as the youngest idol group at the time. The original lineup recorded four indie singles from 2012 to 2013 and released an album with those songs plus three others on March 29, 2014. Onigokko also have a subunit named Haru ☆ Kana, which is now separate from the group. Three of the original members of Onigokko have now graduated and three new members were added on July 12, 2014.
The original lineup in a photo from November 2012....
Members:
Kasaki Ayuki - Born on December 8, 2002
Tsukishima Ai - Born on April 9, 2003
Kasaki Sakura - Born on April 12, 2004
Minami Mikuru - Born on November 6, 2003
Kamiku Nozomi - Born on November 24, 2003
Kozakura Ichigo - Born on July 15, 2005
Trainees:
Kagura Rie - Born in 2003
Kagura Emi - Born in 2006
Motiduki Yumeka - Born on October 23, 2003
Graduated Members:
Tsukishima Meru - Born on June 29, 2001
Kawahara Kana - Born on September 22, 2001
Hoshina Haru - Born on January 6, 2002
Discography:
Onigokko Singles:
“Powerful Girl” (May 3, 2012)
“Cute Land” (July 29, 2012)
“Flying Heart” (December 8, 2012)
"Running Fighter” (June 16, 2013)
Onigokko Album:
“Toy Box” (March 29, 2014)
Haru ☆ Kana...
Haru ☆ Kana Singles:
“Star Road” (July 7, 2013)
“Momomo no Tone” (November 30, 2013)
"Sense Revolution" (September 10, 2014)
Thanks to Pepper for some of the info. 
Check out the website for details. 
I haven't stopped listening to this single since I got it, it's so good.
She did well. So cute!
