The poem gives us two quick images and lets them collide. In a kindergarten classroom, a teacher cuts a paper heart to show children the shape of a triangle. The poem says "the rounds" of a triangle, which is odd, since triangles have corners, not curves. That small slip suggests the lesson is gentle and a little loose, the way teaching very young children often is. Then, in the last two lines, her "good hand" comes down on a roach.
The speaker never says how they feel, and that restraint is part of the effect. The tenderness of the heart is suddenly cut across by something blunt and ordinary: a bug being crushed. "Good hand" is interesting too. It might simply mean her steady or capable hand, or it might hint that the other hand is injured or weaker, which would be a guess from the wording alone. The mood is quietly unsettling and wryly observant, a reminder that gentleness and harshness sit side by side in daily life.