All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Parents or guardians are required to take care of their children and bring them up correctly, whether they are disabled or not. Circumcision is something important that parents or guardian are required to do for their children. Even if the (mentally) disabled children are not eligible for religious obligations, circumcision is physically and psychologically beneficial for them. Additionally, it is an Islamic characteristic, one of the Sunan of the fitrah (acts of natural disposition) and a normal practice of the Prophets, may Allah exalt their mention. Even, some scholars are of the view that circumcision is obligatory for men. Consequently, it is impermissible to abandon it under the pretext that the son is disabled.
Old and recent medical studies have proved the benefits of circumcision for both males and females, particularly for males, towards whom the Sharee'ah asserted this obligation.
Allah Knows best.