My son expressed an interest in math this year, so I bought him some workbooks to do on his own for fun. Whenever he wants to know how to do something new, I teach it to him. I doubt that this hurts him in second grade common core math. The pace of math is so slow at school. But I feel like if kids are curious about something, we might as well teach them. Besides, now DS8 knows how to log onto Kahn Academy and watch videos anyway!