What you have done is truly reprehensible.
It constitutes a serious abuse of the sacrament of baptism.
It also is to commit repeated acts that are outside of ecclesial communion with those who properly and uniquely have the care of souls and the moderation of the liturgy and of the sacraments: namely the pastor of the parish where the grandchildren are canonically domiciled as well as the bishop of the diocese where that parish is located.
Canon Law properly guides the pastor in the decision that is actually his to make – either to admit a child to the sacraments or to defer the administration of the sacraments. Ecclesial governance is to be respected.
It is only in a situation of danger of death and with the impossibility of getting a sacred minister (bishop, priest, or deacon) that a lay person should proceed to baptise an infant. In such a case, there is a liturgical ceremony that is to be conducted as soon as possible afterward to supply, by a sacred minister, the rites that are missing and to duly record the baptism in the sacramental register.
This sort of baptism is NEVER to be done when the reason is that you do not agree with the decision of the pastor or because of parental opposition.
You would grossly compound the offenses you have already committed by doing what you propose. It is the responsibility of the pastor to determine that a child has been adequately prepared to be admitted to first confession and first Communion.
These children are, in fact, not Catholic and have no business receiving either the sacrament of reconciliation or the sacrament of the Eucharist, least of all by subterfuge.
You have put them into an ecclesiastical no-man’s land by your illicit actions. A determination would have to be made as to whether or not the baptisms were valid. Personally, as a priest, I would conditionally baptise them. They would need to be admitted to the Church by a formal act. They would need to be admitted to a programme to properly prepare them to receive the remaining sacraments of initiation: first reconciliation, first Communion, and Confirmation through their proper canonical parish.
What you are doing is not only grievously injurious to your own soul and destructive to your own relationship to the Church, you are setting up your grandchildren for the eventual discovery that they have no sacramental records – either when they seek confirmation, marriage, or religious profession or holy order – and no affiliation with the Church.
Now…Beyond this canonical quagmire is the actual situation of the parents. All else being equal, and where it truly concerns infants, the consent of one parent is sufficient relative to Canon 868. That presumes, however, that the other parent is not adamantly opposed. Given the presence of a divorce, and above all if there is a court imposed custody arrangement, neither the parish priest (nor the bishop) will act in a manner that contravenes the finding of civil authorities relative to religious accommodation…nor should they; the diocese would risk being justly found in contempt of court.
IF the children wish to be received into the Catholic Church and IF the custodial parent was to consent, that needs to be worked out with the canonical pastor of these children, who needs to be informed of the illicit baptisms as that knowledge will alter the procedure of what the priest would need to do to resolve this most unfortunate situation. If the custodial parent remains opposed, the children will need to wait until they have reached adulthood. There, too, the priest would need to be informed of the illicit baptism.