The Holy Thursday foot-washing is not a Sacrament (and the Church cannot change the matter of a Sacrament ---- ever: it does not have the authority). It is a (small s) "sacramental," which means the Pope certainly has the option to include women, without implying the whole structure of sex, gender and Natural Law is going to come crashing down on our heads!!
¡Viva Papa Francisco!
That said, the GIRM clearly uses 'viri', that is 'men' for the Washing of the Feet. This is just an option for Holy Thursday and need not be done at all. However, if one is going to do it, one should stick to the General Instructions. If the leader does not, it simply gives license to all to do exactly as they please.
Further, washing the feet of a 12 year old Muslim girl might not be the wisest thing to do.
And the real identity of each soul is not man or woman as manifested in this earthly realm.
In the Kingdom of God we will have our real, eternal identities which may be very different from the guises we now appear in.
A Muslim girl is not eternall a Muslim girl. Underneath or rather, inside, the earthly tent, lives the eternal soul.