Though Adam sinned, he entered heaven. So then his descendants should also enter heaven by the grace and Mercy of God right?

Not everyone will enter heaven. Souls of those who die deprived of sanctifying grace will immediately enter hell. Sanctifying grace is a habitual gift, a stable and supernatural disposition that perfects the soul itself to enable it to live with God and to act by his love (CCC 2000). It’s ordinarily gifted through the sacrament of Baptism but is lost with the very first mortal sin committed after baptism. Still, it can be regained by the sacrament of confession and the sacrament of anointing of the sick (if the person is unable to regain it through the sacrament of confession). Under extraordinary circumstances baptism by desire and baptism by blood bestows this grace. As an extraordinary means this can be also regained, if lost by mortal sin, by an act of perfect contrition.

Adam’s admission into heaven, does not assure his descendant’s admission to heaven. Adam did not enter heaven in his sinful nature. He did because he repented for his sins, lived a life of penance and died with sanctifying grace in his soul. When Jesus descended to Sheol, he raised all the just who died before Him, including Adam. 

Jesus has redeemed the world, but for a person to attain salvation he needs to accept the redemption. Redemption and Salvation have slightly different meanings. While redemption is universal, salvation is personal. Everyone has the free will to accept or reject the sanctifying grace offered by the Christ and subjected to the presence and absence of sanctifying grace at the time of death he is destined accordingly to heaven and hell.  Let us get familiar with the ordinary and extraordinary means of restoring sanctifying grace and practice it in everyday life.