I'm still certain the term impious could be applied to any without a proper respect of God, whether it be from ignorance or choice.
For the record, I would call a dog impious, simply because it cannot be pious.

Additionally, from Quam Singulari
A full and perfect knowledge of Christian doctrine is not necessary either for First Confession or for First Communion. Afterwards, however, the child will be obliged to learn gradually the entire Catechism according to his ability.
I think, if a full understanding is not necessary for first confession or communion, but we are obliged afterward to learn the entire catechism, it would contain that which is necessary for a full and perfect knowledge.
I assume you'll show how this is not binding, but the 7 year age is.