What is happening here?
8 + 5 or 13 - 7: this is where children first have to count across the 10. Many kids count on their fingers, lose track, or give up after several tries. That is normal and not a sign of a learning difficulty.
What helps at home
The "fill up to 10" trick works well. With 8 + 5 you ask "How many does the 8 need to reach 10?" (2). Then 3 are left from the 5, so 10 + 3 = 13. Saying this out loud helps it stick.
Practice this strategy step by step with small problems before moving on to harder ones. Two or three problems a day are better than one long practice session.
Materials
Snap cubes in rows of ten or a counting frame make the jump visible. A simple number line on paper (0 to 20) that the child can hop along also helps them understand it.