A math term for subtraction is difference. Practice subtraction at home when watching football or playing corn hole. Ask, Who is winning? By how much? What is the difference in the scores? After they have given an answer, ask how they got it (don’t indicate yet whether they got it right or wrong). Then share how you arrived at it, and point out any errors in their thinking. Congratulate them on anything about their solution that was right, even if the answer was wrong.
Example:
Ping pong scores: 19 to 7
Parent: “How much are you winning by?”
Student: “11”
Parent: “How do you know?” “Prove it.”
Student: “I added up from 7.”
Parent: “Show me.”
Student: [on their fingers] “8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18.. oh wait, it’s 12.”
Parent: “Good subtracting. My strategy was to take 19-10=9. Then I subtracted 2 more, making a difference of 12. Your turn.”
Notice that instead of chiding them for using their fingers, you are gently showing them a more efficient way by sharing your strategy. Children gradually gravitate towards quicker, easier strategies (don’t we all?).