Is what happened between two children (one 8 or 9 years old and the other 6 or 7 years old) considered zinā, and will their repentance be accepted, and will the repentance of one who practiced masturbation out of ignorance of its prohibition be accepted?

Allah accepts sincere repentance for any sin, including zina (fornication/adultery), in accordance with His saying: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" (Quran 39:53) And the one who repents from a sin is like one who has no sin. Sins committed before puberty are not held accountable, because the pen of accountability does not run for a servant until after they reach puberty.