It should not be permissable for the bodies of children to be branded, or modified in any way when there is no overwhelming medical justification.
Indeed, coerced genital modification of minors - along with other forms of religiously motivated branding of the body - goes against the very spirit of religious freedom, in that it does not protect the right of the individual to make their own religious choice and permits adults to endorse the religious branding of a child before such an age as the child may make their own personal choices regarding their religious, philosophical, and political beliefs.
We should free the individual from the tyrrany of ill-conceived and damaging cultural practices. I believe that society's common laws are paramount and that religious laws are secondary. Religions must operate within the boundaries of society's common laws. In order to dissent on any one matter, a religion must work within the system of the society that plays host to it. And as long as that dissent remains unsuccessful, such a religion must honor and abide by those prevailing rules or restrictions against which they dissent.
The outlawing of medically non-essential body modification (including religiously motivated genital modification) would be a victory for children of both Jewish and Islamic parents, since it would liberate the individuals of such religious upbringing from the tyrrany of religious practices that choose to brand their bodies, without regard to the possibility that they may later grow to choose different religious beliefs than their parents - beliefs under which their religious brandings may not be necessary, or may even be unacceptable.
Therefore, I believe that at the age at which a society considers it acceptable for individuals to become sexually active, only at that age should that society also empower individuals to consent to non-medical genital modification. It does not seem to make any sense to permit an individual to consent to sex, but still deny the individual the right to consent to genital modification. So, this may mean that children would be permitted to consent to genital modification at a reasonably early age in many cultures.
It seems like the level of good judgement that society regards as being necessary for an individual to engage in consensual sexual activity should be equally suited to any decision the individual may wish to make to have their genitals modified in any cosmetic manner permissable by society, and not deemed to be substantially self-destructive and evidence of the individual posing a threat to their own wellbeing.