A Forbidden Time [OVA]
This is an adult anime.
If you’re looking for a non adult TV anime titled “Kodomo no Jikan”, see http://www.animenfo.com/animetitle,3988,alrtcc,kodomo_no_jikan.html
A high school girl Saki was asked to take care of neighbor children while the parents were absent. Saki thought it as an easy job, but precocious children began to make her their toy. Since then, she is being sexually abused by them.