The Sacrament of Baptism does several things, among them are the cleansing of original and personal sin and imparting of membership in the Catholic Church.
As we all know, some non-Catholics, both heretics and schismatics, baptize infants. Assuming such baptisms are valid, does that mean that the infants are Catholic, despite what their parents say, until they reach the age of reason and reject the Church by accepting the heretical of schismatic group?
To put it another way: a Catholic couple has their baby baptized in a Catholic church on the same day as a Lutheran couple in a Lutheran church (assume the Lutheran baptism was valid, which it can be according to Church teaching). Both children received the Sacrament of Baptism, so shouldn't both receive all of its effects, including cleansing of sin and membership in the Church, until such time that he or she actually rejects Church membership?