Is a killer imprisoned for 15 years until the children of the victim can discern and their opinion is taken, or is the opinion of the adult guardians of the blood taken? And what would be the killer's situation during this period if there is a wait until the children reach maturity?

If one of the adult and mature guardians of the blood forgives the retribution (qisas), then the entire qisas is dropped, because qisas is indivisible. It is irrelevant whether the forgiveness is in exchange for a blood money (diyya) or not. As for those guardians who did not forgive, they are entitled to their share of the diyya from the killer's wealth or from the Muslim treasury (Bayt al-Mal). There is no waiting for minors to reach maturity to forgive; rather, they accept the diyya. However, if the adults insist on qisas, they are permitted to carry out the execution without waiting for the minors to reach maturity, according to the Hanafis, a statement among the Malikis, and one of two statements from Ahmad. The perpetrator may be imprisoned until the minor reaches maturity, and this is the madhhab (definition) of the Shafi'is and Hanbalis.