You say you are Protestant in your profile. If so, and if your children/grandchildren are non Catholic, this was not an illicit baptism. You are correct that the priest is the ordinary minister of baptism in the Catholic Church. A Catholic should not baptize a child outside danger of death, and certainly not without parental permission.
To the degree you did this behind the parents’ backs or against their wishes, certainly you may have some culpability for sin. But if you did it with their knowledge or at least implicit approval then it was neither sinful nor illicit as none of you are members of the Catholic Church.
And while it would not be recorded in the Catholic baptismal register if the parties are all non Catholics, you should certainly record it somewhere. Maybe a family bible or on paper kept with birth certificates. Note the names of the baptized, who performed the baptism, any witnesses, date, place and manner – the formula used and pouring of water.