By “child” I do not mean a minor, but simply a son or daughter (of adult age).
I remember reading somewhere that a person’s sponsor cannot be someone in the direct line, but the context was whether a parent/grandparent can be the child’s sponsor. My question is vice versa.
Interesting question. I cannot see that this is addressed by canon law.
Can. 892 As far as possible the person to be confirmed is to have a sponsor. The sponsor’s function is to take care that the person confirmed behaves as a true witness of Christ and faithfully fulfills the duties inherent in this sacrament.
Can. 893 §1 A person who would undertake the office of sponsor must fulfill the conditions mentioned in can. 874.
§2 It is desirable that the sponsor chosen be the one who undertook this role at baptism.
Can. 874 §1 To be admitted to undertake the office of sponsor, a person must:
1° be appointed by the candidate for baptism, or by the parents or whoever stands in their place, or failing these, by the parish priest or the minister; to be appointed the person must be suitable for this role and have the intention of fulfilling it;
2° be not less than sixteen years of age, unless a different age has been stipulated by the diocesan Bishop, or unless the parish priest or the minister considers that there is a just reason for an exception to be made;
3° be a catholic who has been confirmed and has received the blessed Eucharist, and who lives a life of faith which befits the role to be undertaken;
4° not labour under a canonical penalty, whether imposed or declared;
5° not be either the father or the mother of the person to be baptised.
§2 A baptised person who belongs to a non-catholic ecclesial community may be admitted only in company with a catholic sponsor, and then simply as a witness to the baptism.