I agree that not all babies (nor all children, nor all adults) are elect. We are all conceived with an inherent sin nature; not one is righteous, no not one.
No one here has suggested that "not all babies...are elect" since there is no possible way to know this. We are not able, particularly from the statement in WCF 10.3 (about "elect infants dying in infancy") to draw the conclusion either that "all infants who die in infancy are elect," or "some infants who die in infancy are not elect." The statement is simply telling us that the elect are saved, whether as an infant "incapable of being outwardly called by the ministry of the Word" or "all other elect persons" (someone mentally disabled, e.g.), who are likewise incapable of being outwardly called.
It is the case that the Princetonians went in the direction of arguing that all who died in infancy were presumed elect. I'm not sure that there's the clearest warrant for that in the positive, but there is certainly no reason to assume the opposite--that some clearly go to hell. We have no biblical evidence that any dying in infancy go to hell. Their being conceived and born in sin is no clue to that, since we all are and it's all of His grace (As Ray noted), for infants or adults, the difference being that adults are ordinarily capable of being outwardly called and thus we look for faith and repentance that we cannot see clearly in an infant or other person not able to manifest such.
What I can most clearly and comfortably rest in is that great declaration of the Synod of Dort (1.17) that says that Christian parents ought not to doubt the election and salvation of those that God calls out of this life in infancy. Bottom line: if I am speaking to unbelievers, I would never suggest that their child "may have" gone to hell. Rather, I would ask them if they are ready to meet Him whom their child has met and offer them comfort with respect to their child (you can rest him or her in the hands of the Lord and you need to be resting in Christ alone). To Christian parents: You can rest assured that your child is safe in His arms.
In fact, I tell anyone worrying about departed relatives ("I'm not sure that he knew the Lord;" or flatly, "She was not a Christian"): we entrust all those who die to Him who does all things well. "If they were His," I say, "they're with Him, whether you saw them testify to faith in Him or not." We rest in Him, knowing that He does all things well.
Now to the OP: We will be just what we need to be there. It makes no more sense that we would be eternally an embryo or infant than that we would be perpetually aged. So we can't say more than that but we know that there we will lack no joys or blessings that we have here; whatever is different there than here (e.g., neither marrying nor giving in marriage), we will lack nothing there than we enjoy here (every joy of marriage here will be perfected and transcended there).
Peace,
Alan