As far as my experience goes, it appears to me that believers generally have expected far too little of present fruit upon their labours among children. There has been a hoping that the Lord some day or other would own the instruction which they give to children, and would answer at some time or other, though after many years only, the prayers which they offer up on their behalf. Now, while such passages as Proverbs xxii.6, Ecclesiastes xi.1, Galatians vi.9, 1 Cor. xv.58, give unto us assurance not merely respecting everything which we do for the Lord, in general, but also respecting bringing up children in the fear of the Lord, in particular, that our labour is not in vain in the Lord; yet we have to guard against abusing such passages, by thinking it a matter of little moment whether we see present fruit or not; but, on the contrary, we should give the Lord no rest till we see present fruit, and therefore, in persevering, yet submissive, prayer, we should make known our requests unto God.
I add, as an encouragement to believers who labour among children, that during the last two years seventeen other young persons or children, have been received into fellowship, among us, and that I am looking out now for many more to be converted, and that not merely of the orphans, but of the Sunday and day-school children."
From George Müller's Narratives.