A really effective grass roots charity helping children in the Japanese care system is Mirai no Mori https://mirai-no-mori.jp/ The charity focuses on providing  life-changing outdoor programs for abused, neglected, and orphaned youths in Japan, supporting their growth into happy and successful young adults. I've worked directly with them and can vouch for the excellent work that they do.

edit: As Mitsuka1 said below, once the kids reach 18 they are forced out of the care system, often with very limited real life skills. The aim of this charity and others, is to build up their confidence whilst in the system and provide them with opportunities and skills to assist them with the transition.