The Church gives us guidelines indirectly. The Code of Canon Law says, "Merely ecclesiastical laws bind those who have been baptized in the Catholic Church or received into it, possess the efficient use of reason, and, unless the law expressly provides otherwise, have completed seven years of age" (CIC 11).
So there is no obligation on the child’s part to go until completion of his seventh year.
But, by their baptism, children also have certain rights: "Since they are called by baptism to lead a life in keeping with the teaching of the gospel, the Christian faithful have the right to a Christian education by which they are to be instructed properly to strive for the maturity of the human person and at the same time to know and live the mystery of salvation" (CIC 217).
"According to their own vocation, those who live in the marital state are bound by a special duty to work through marriage and the family to build up the people of God. Since they have given life to their children, parents have a most grave obligation and possess the right to educate them. Therefore, it is for Christian parents particularly to take care of the Christian education of their children according to the doctrine handed on by the Church" (CIC 226).
So it is the primary obligation of the parents to educate their children in the faith and prepare them for a eucharistic life, and it is the children’s right to receive this education. Obviously, before the Sunday obligation takes effect for the child, he already should be participating regularly in the eucharistic celebration to the extent that he is able. How this is accomplished in each family will vary.
From an early age—especially from the time they are baptized—it is appropriate that children be included in the eucharistic celebration. Yet not all children are ready or willing. In the meantime, the parents hopefully will be praying for and with their children and preparing them for regular Mass attendance.
Many parents with infants find it very difficult to bring them to Mass, especially if a toddler is also in tow. A single parent will find this especially difficult. But the goal is to get them coming as soon as they are able, and if they are not able yet, get them ready by familiarizing them with the church, maybe by going for short visits and gradually extending those visits. Going to church should be a privilege and something they want to do.
While children cannot fully understand all that goes on at Mass, they can participate in some ways. They can learn to genuflect and bow. They can place the envelope in the basket. They can sing, and the family can practice songs at home. If the Our Father is said regularly at home, then they will look forward to saying it. The same holds true for the sign of peace.
The Congregation for Divine Worship’s Directory for Masses with Children states: "By reason of the duty in conscience freely accepted at the baptism of their children, parents are bound to teach them gradually how to pray. This they do by praying with them each day and by introducing them to prayers said privately. If children, prepared in this way even from their early years, take part in the Mass with their family when they wish, they will easily begin to sing and to pray in the liturgical community and indeed will already have some initial idea of the eucharistic mystery. . . .Infants who as yet are unable or unwilling to take part in the Mass may be brought in at the end of Mass to be blessed together with the rest of the community. This may be done, for example, if parish helpers have been taking care of them in separate areas" (DMC 10, 16).
The goal is to help our children to participate fully in the eucharistic life. Choose a way to get them there that works best for your family.