Q: A shahid that died before puberty walks straight to jannat, so then how does that person intercede for the allowed 70 people to be interceded on their behalf?
A: There is no contradiction between the both i.e. Being entered into Paradise and interceding for people. Just as it is possible for a person living in South Africa to intercede on behalf of someone who is living in America.
And Allah Ta’ala (الله تعالى) knows best.
Checked & Approved: