Their question is addressed in the Compendium to the Catechism of the Catholic Church, and it is actually a sensible question, considering the effects of receiving Holy Communion. The answer is that while it is true that Holy Communion wipes away venial sin, making the Sacrament of Penance an absolute necessity only the in the case of mortal sin, the Church nevertheless strongly encourages the confession of venial sins as well in order to receive the graces inherent in the Sacrament of Penance.
I would like to start with 292 from the Compendium, because this truth they learned in grade school may be the source of your parents’ confusion. Your answer can simply be this: “Yes, Holy Communion wipes away venial sin, but that is not all confession does. I do better when I go to confession. I treat other people better, I feel God’s love, I have the advantage that I have someone I actually can trust entirely to know all my deepest worst secrets and instead of judging me he helps me turn away from my worst side and then he forgets about it! Who of you can say that? So while Holy Communion does have that effect, Fr. X is a very good confessor, and I really recommend you come with me some time. How about during Lent?” And then start working on them a little when they bring it up! (By the way, if you do not have the Compendium or the Catechism, they are posted online in their entirety.)
292. What are the fruits of Holy Communion?
Holy Communion increases our union with Christ and with his Church. It preserves and renews the life of grace received at Baptism and Confirmation and makes us grow in love for our neighbor. It strengthens us in charity, wipes away venial sins and preserves us from mortal sin in the future.
306. Why can venial sins also be the object of sacramental confession?
The confession of venial sins is strongly recommended by the Church, even if this is not strictly necessary, because it helps us to form a correct conscience and to fight against evil tendencies. It allows us to be healed by Christ and to progress in the life of the Spirit.
That is, from the Catechism:
**1448 **Beneath the changes in discipline and celebration that this sacrament has undergone over the centuries, the same fundamental structure is to be discerned. It comprises two equally essential elements: on the one hand, the acts of the man who undergoes conversion through the action of the Holy Spirit: namely, contrition, confession, and satisfaction; on the other, God’s action through the intervention of the Church. the Church, who through the bishop and his priests forgives sins in the name of Jesus Christ and determines the manner of satisfaction, also prays for the sinner and does penance with him. Thus the sinner is healed and re-established in ecclesial communion.
This is also in the Catechism:
1468 "The whole power of the sacrament of Penance consists in restoring us to God’s grace and joining us with him in an intimate friendship."73 Reconciliation with God is thus the purpose and effect of this sacrament. For those who receive the sacrament of Penance with contrite heart and religious disposition, reconciliation "is usually followed by peace and serenity of conscience with strong spiritual consolation."74 Indeed the sacrament of Reconciliation with God brings about a true “spiritual resurrection,” restoration of the dignity and blessings of the life of the children of God, of which the most precious is friendship with God.75