What's this music in Darling in the FranXX? It's Hiro and Zero Two during their childhoods when they are separated by the guards : 


https://youtu.be/gqvZJMcIxHA?t=66 . (spoil)

The OST start at 1 minute and 6 seconds.

Also, this OST is in the 13th episode and start at 21 minutes and 17 seconds

Thank you for your help !
After a few days of research, I found it : this is the 4th ending of Darling in the FranXX : Hitori

https://www.youtube.com/watch?v=ltXFpzHxcoY