And since you accept "baptism of desire" why the concern? ;-)
(1) Baptism of desire requires perfect contrition or love of God above all things, whereas baptism of water requires only attrition, or sorrow for sins because of the fear of punishment;
(2) Children cannot be saved by baptism of desire;
(3) Simulation of a sacrament, even by one invincibly ignorant of the required matter and form, is a grave sin; moreover, it is harmful to the dignity of the sacrament;
(4) Baptism of desire does not remit the temporal punishment due to sin;
(5) Baptism of desire does not imprint the baptismal character, and it is fitting that men "be conformed to Christ's other members by receiving the character" (St. Thomas, III q. 68 a. 1). By a similar reason, "in danger of death children should be confirmed even if they have not yet attained the age of discretion" (CCC 1307), even though the reception of this sacrament is, for infants, in no way necessary for salvation.
If you consult St. Alphonsus Liguori's "Theologia Moralis", you will find that he treats extensively of the required form and matter of baptism of water, as regards both lawfulness and validity, directly after affirming the "de fide" status of baptism of desire.