Adds the tops and the bottoms
Your child adds fractions the way they add whole numbers: top plus top, bottom plus bottom. One half plus one third becomes two fifths. The answer is smaller than the half they started with, which is the giveaway.
What the teacher saw: adding one half and one third on strips of sixths, they shaded 2 of 6, the two top numbers, ignoring the sizes.
From the teacher. Next class works on adding fractions with different-sized parts. Read and approved by a person before it reaches you.