It is the blacksmith himself who attracts the children after school to stop by and watch him work. The speaker states, "They love to see the flaming forge, / And hear the bellowsroar, / And watch the burning sparks that fly" (21-23). Longfellow's imagery also attracts the reader with the senses of sight, sound, and touch--the flames, the roar, and the burn. As well, one can imagine that the children are also attracted by the might of the blacksmith, as that is the description that starts the poem:
The smith, a mighty man is he,
With large and sinewy hands;
And the muscles of his brawny arms
Are strong as iron bands. (3-6)
Children are often impressed by feats of strength and bravery, and a man who works with heat and fire to bend and mold metal items would be the very picture of those qualities.