You will receive neither eternal reward nor eternal damnation but might face eternal resentment if your son and daughter-in-law discover what you've been up to. They may well consider the religious guidance of their children to be a parental prerogative, reasonably enough. For anyone to intervene, even a well-intentioned family member, might confuse the kids (if they are old enough to recall your idiosyncratic ritual) and undermine their relationship with their parents. One ethical guideline: a description of conduct that begins with "I snuck" is apt to raise doubts.

To a secular person, an incantation whispered over an infant is harmless nonsense. But as a believer, you should be aware that Catholicism regards baptism as a sacrament to be performed by a priest except in an emergency, when anyone may step in. But "emergency" is generally held to mean the imminent death of the child, not a doctrinal dispute with the parents.