Whose to say that an aborted or miscarried baby is not given the chance to receive baptism of desire the last second of its life...
In order to receive the grace of baptism in the manner you are mentioning, it is necessary to have the use of the intellect and the will. This is not the case with infants, etc.
Essentially, you are positing the occurrence of a miracle (for it would definitely be one in such a case) that allows an infant to use his (inoperative) intellect and will before his soul is separated from his body.
Why not posit that God, WHO COULD, works a miracle in the case of every single man, saving them all? An obstinate will is just as easily overcome by omnipotence as is an inoperative one.
What about the fact that God gave enlightened the babies killed by Herod with supernatural faith and thereby gave them the chance to accept their martyrdom for the faith?
I also know of one story of a 2 year old who died in sanctity at that age and who possessed more supernatural faith than most Catholic adults - I read about her life story in an article but forgot her name and where I read it. What she understood about the faith and suffering no normal 2 year old can.
I am not saying that every aborted or miscarried child ends up being given the chance to accept the faith - just like not every non-Catholic is given the chance the last moment of their life. I think who God decides to give the grace of baptism of desire the last second of their life might depend on the prayers of the child's parents or loved one (such as Fr. Augustine's case who prayed for his mother) or the fact that, had the child not died or been killed, one or both of the parents would have baptised the child.
There is also the fact that God knows from all eternity who belong to Him. Recall this Sunday's gospel reading where Jesus said:
"I am the Good Shepard, and I know Mine and Mine know Me...And other sheep I have that are not of this fold. Them also I must bring, and they shall hear My voice, and there shall be one fold and one Shepherd."
This is the reason why God gives miraculous graces of conversion to some sinners and non-Catholics while to others he does not? Because there are those who are "predestined" to belong to Him that He must call back to the fold. So for those babies that are of His sheep but that would be killed before they can enter His fold, He can still call to Him by granting the opportunity of baptism of desire before they get killed.