Unborn and Heaven

On the first question, Reformed theologians have not been entirely unified.
Some, particularly the old Princeton theologians, have argued that while imputed sin from Adam is sufficient to make someone a sinner and to render him spiritually dead, it does not merit punishment. Because infants have not committed any sins (e.g. Rom. 5:14), there is nothing for which they can be punished. Because there is nothing for which they can be punished, they cannot go to hell. Finally, since there are only two options (heaven and hell), God regenerates infants who die so that they may enter heaven. A shortcoming of this position is that while infants gain heaven through God's mercy and grace, they seem to escape hell by virtue of some merit of their own.

Others have argued that imputed sin entails real imputed guilt, and that imputed guilt is real guilt (the infant is really guilty of having sinned, even though he has not actually sinned). Because the infant is really guilty of sin, he may be justly punished for this sin. If it were not just to punish someone for imputed sin, then Jesus could not justly have suffered for the sins of men. Thus, infants who die may justly be sent to hell.

Most who believe that infants may justly be sent to hell, however, do not believe that all infants who die actually go to hell. Rather, most believe in the existence of elect infants -- infants whom God sovereignly regenerates and saves, despite the fact that they deserve hell. That people can be elect from infancy, and even from the womb, appears to be demonstrated by John the Baptist in Luke 1:44. This does not necessarily imply that all infants who die are elect, though some Reformed theologians have also held this position.

A good argument can be made that God shows particular favor upon covenant children who die, so that believers may have more confidence than others that their children who die in infancy are among the elect. This idea is implied by the fact that God does not treat his covenant people with the same strictness with which he treats others. Rather, with his covenant people, he is slow to anger and quick to show mercy. He also has a special love for the children of believers (Ps. 103:17). Further, God's love for believers inclines him to be good to believers, and the Bible tells us that children are God's gift to believers (Ps. 127:3). This implies that one way that God blesses covenant members is by treating their children with mercy (compare Gen. 26:24; 1 Kings 11:12). Moreover, the ideal blessing which God describes for his people includes the lives and blessing of their children (Isa. 65:18-23), creating for us an expectation that God will be good to our children even when they die in infancy (i.e. that he will save them). This is my position.

On your second and third questions, I don't believe there is such a thing as an age of accountability under which we can be certain that a child goes to heaven. I wish the Bible taught such a doctrine, but I am not persuaded that it does. One thing that is clear in Scripture, however, is that if any person consciously rejects Christ, he or she will not be in heaven. Presumably, children raised in Christian homes will be taught the gospel from their earliest days. If they reject this witness, then I hold out very little hope that they are saved regardless of their age. However, most children believe what their parents teach them, including the gospel, and my hope would be that most Christians teach their children the gospel. In the hopefully rare cases in which parents fail to teach their children the gospel, and in which the children are older (and die), in my opinion the children are less likely (not unlikely) to be elect. If children do not understand the gospel (due to age, infirmity, etc.), then I would categorize them as infants.

If we may speak of an age of accountability, I would define that as the age at which a child can comprehend the gospel, not as the age at which a child becomes accountable before God -- we are always accountable before God. If the child has been taught the gospel, then at that age the child must either receive the gospel, or reject it (at least temporarily -- he/she may receive it later). If the child has not been taught the gospel, then he/she is in peril, and his/her parents are remiss. But there is uncertainty in all of this -- we cannot know the hearts of our children or read their minds. The most I feel comfortable saying is that the older a child is, the more likely he/she has reached that elusive age of accountability.
In all of this, it is important to remember that God saves all his elect. If a covenant child is elect, God will not fail either to bring that child to salvation. If a child is not elect, then that child will be punished justly for his or her sins. Because God loves his people, in my opinion the former is much more likely to be the case with the children of believers.