I don't have kids, so I can't offer anything from the perspective of a parent. But I do have the experience of being the child of highly educated, intelligent parents who put a lot of pressure on me to succeed academically. My dad was a literal genius. I was merely a good student, considerably above average in most subjects, but not a genius or 1600 SAT material. My recollections and suggestions follow. (A lot of this is Parenting 101, but as smart as they were, my parent's weren't always great at this.)
1) While helping your kids study or do homework, try not to show impatience or anger if they are slow to catch on to something. My dad tried to be patient with me, but still there was enough of a gulf between us that it was apparent that he was frustrated that I wasn't getting it. It made me feel, well, dumb. Which, in comparison to him, I was. But he had at least 40 IQ points on me.
2) Don't push your kids into advanced classes or grades that they aren't prepared for. In middle school my parents lobbied hard to get me into "GT"-level classes for English, math, science, etc. Which was OK in the case of English and science subjects, I did well at those. But I was not, and still am not, adept at math, especially abstract math. I was hopelessly lost in that math class, and it was frustrating and embarrassing, and led to further problems described in my point 3 below.
3) If you put too much pressure on a kid to excel with grades, it can have counterproductive results. For example, I was failing the aforementioned math class. The right thing to do would have been to tell my parents/teacher that I couldn't do it, or that I needed extra help, or something along those lines. But I was much too afraid about my parent's reaction to handle it that way. Instead, I tried to cover up my poor grades as long as I could. (I altered my report card.) Giving my parents a report card with a D or F on it was just unthinkable. Of course I eventually got caught, and I even knew I was pretty likely to get caught--but at the time I didn't really feel like I had much to lose.