But Jesus called them to Him, saying, “Let the children come to Me, and do not hinder them; for to such belongs the kingdom of God.” – Luke 18:16

The mass of people in London go to no place of worship now; the old habit of attending church or chapel seems to have been given up; but the people will still let the children go to Sunday-school, even if they do it from no better motive than that of getting them out of the way in the afternoon, or in order that the house may be quiet without them. Anyhow, if you open a school anywhere in London, you can quickly get it filled with children. If you cannot do one thing, do another. If you cannot reach the fathers and the mothers, though you should earnestly try to get at them, yet, if you can reach the children. Take care that you lose no opportunity of teaching them the things of God…While yet the clay is soft, mould it for God. May the Lord Himself help you, dear Sunday-school teachers, and others who labour amongst the children, to do your work right well! Nobly are you serving your own generation, and the generation to follow. This is the work that lies nearest to you; seek to accomplish it; and “whatsoever thy hand findeth to do, do it with thy might.” …The salvation of the children ought to be sought with double diligence, for they will last the longest. If a man of sixty or seventy is converted, he will have only a short time for serving God here; for he will soon be gone. If a child is converted, a long life of usefulness may enrich the church of God. Remember, too, that those who are converted when children usually make the best saints. Therefore, look after the children. ~ C.H. Spurgeon

https://www.blueletterbible.org/comm/spurgeon_charles/sermons/2243.cfm