A baptised Protestant child under the age of 7 most likely would be saved. Through Baptism we are joined with the Church(and even Baptisms by heretics can be valid), it's only by the embracement of heresy that Protestants are separated from the Church. A child under 7 having not achieved the age of reason would be spared the sin of heresy AFAIK and therefore still count as a member of the Church. Although that would be in spite of the child being a Protestant.
Please correct me if I'm wrong on this.
Yes, they are Catholics until they profess heresy, since they have received the Sacrament of Baptism. As for the requirement to receive Holy Eucharist, that's considered necessary by necessity of precept but not absolutely necessary for salvation. Adherence to heresy is one of the things that excludes from membership in the Church (cf. St. Robert Bellarmine's criteria). Now, is it possible that an invincibly ignorance 10-year-old could not pertinaciously adhere to heresy and be saved? Sure, I think so. But then he's still a Catholic in material heresy. Once the nature of this heresy, however, touches the point where the person no longer has the correct formal motive of faith, the heresy transitions into formal heresy ... even if there's no active sin against faith.
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