Train them to be faithful and regular in attending church and the Lord’s Supper.
Tell them of the duty and privilege of going to Church, and joining in the prayers of the congregation.
Tell them that wherever the Lord's people are gathered together, there the Lord Jesus is present in a special way, and that those who are absent must expect, like the Apostle Thomas, to miss out on a blessing. Tell them of the importance of hearing the Word of God preached, and that it is God's ordained way of converting, sanctifying, and building up the souls of men. Tell them how the Apostle Paul commands us not to "give up meeting together, as some are in the habit of doing, but to encourage one another—and all the more as we see the Day approaching." [Hebrews 10:25]
It is a sad sight in a church when nobody comes to the Lord's Table but the older people, and the young men and the young women all turn away. But it is a sadder sight still when no children are to be seen in a church, except those who come to the Sunday School, and are often obliged to attend. Let none of this guilt lie at your doors. There are many boys and girls in every city, besides those who come to Sunday School, and you who are their parents and friends should see to it that they come with you to church.
Do not allow them to grow up with a habit of making vain excuses for not coming. Make them clearly understand, that so long as they are under your roof, it is the rule of your house for every one in good health to honor the Lord on the Lord's day, and that you believe that the healthy person who refuses to go to church on the Lord’s Day brings great harm to his soul.
Also see to it, if it can be arranged, that your children go with you to church, and sit near you when they are there. To go to church is one thing, but to behave well at church is quite another. And believe me, there is no guarantee for good behavior like that of having them under your own watchful eye.
The minds of young people are easily distracted, and their attention lost, and every possible means should be used to counteract this. I do not like to see them coming to church by themselves—they often get into bad company, and so learn more evil on the Lord's day than in all the rest of the week. Neither do I like to see what I call "a young people's corner" in a church. They often catch habits of inattention and irreverence there, which it takes years to unlearn, if they are ever unlearned at all. What I like to see is a whole family sitting together, old and young, side by side—men, women, and children, serving God as a family.
But there are some who say that it is useless to urge children to attend church and the Lord’s Supper, because they are too young to understand them.
Do not listen to such reasoning. I find no such doctrine in the Old Testament. When Moses goes before Pharaoh, I observe that he says, "We will go with our young and old, with our sons and daughters, because we are to celebrate a festival to the LORD." [Exodus 10:9] When Joshua read God’s Law, I notice that the Bible says, "There was not a word of all that Moses had commanded that Joshua did not read to the whole assembly of Israel, including the women and children." [Joshua 8:35] And when I turn to the New Testament, I find children mentioned there as partaking in public acts of religion as well as in the Old Testament. When Paul was leaving the disciples at Tyre for the last time, he said, "We left and continued on our way. All the disciples and their wives and children accompanied us out of the city, and there on the beach we knelt to pray." [Acts 21:5]
Samuel, in the days of his childhood, appears to have ministered to the Lord some time before he really knew Him, the Bible says, "Samuel did not yet know the LORD: The word of the LORD had not yet been revealed to him." [1 Samuel 3:7] The Apostles themselves do not seem to have understood all that our Lord said at the time that it was spoken: "At first his disciples did not understand all this. Only after Jesus was glorified did they realize that these things had been written about Him." [John 12:16]
Parents, comfort your minds with these examples. Do not be discouraged because your children do not see the full value of church and the Lord’s Supper now. Just train them to have a habit of regular attendance. Set it before their minds as a high, holy, and solemn duty, and believe me, the day will very likely come when they will bless you for your efforts.
The book "The Duties of Parents" can be found here: J.C. Ryle