I strongly believe that difficulty with a certain subject does NOT mean that you or your child are not cut out for a particular field. If you're interested in something, understanding will eventually come to you. The trick is to help it out.
The visit to Q's lab was always my favorite part of a James Bond movie. In elementary school, I wanted to be an inventor (which I believed had a much more concrete job description than it really does, definitely involving power tools and Tesla coils). As I grew up, I set my heart on engineering. But I was not one of those kids who loved school. Science and math did not come easily to me. I struggled for a long time to maintain a B average in STEM courses. Theorems and lemmas went in one ear and out the other, no matter how hard I tried to memorize them. Unfortunately, I wanted to be an engineer anyway.
It took me a while--until my 20s--to realize that my problem wasn't a neurological deficiency or some sort of genetic bias (my parents were both English majors); the typical classroom environment just does not do it for me. Reading a theorem and understanding it can be two very different things. I need to experiment, to observe theory in action, in order to learn effectively. A little trial and error can illustrate the "why" of a theorem or principle that seems nonsensical at first glance. It was this realization that made all the difference in my education.
I still cannot memorize theorems or the names of the people who came up with them. If someone says "Oh, I used the Blahblah Blah Theorem," I have to Google it. The difference is that now I have the learning tools I need to understand new material. I know it sounds corny, but I use "arts and crafts" to help myself understand new material all the time. I go through crayons and duct tape so fast I've started buying them in bulk. I recently had some trouble understanding the mechanics of a system we were talking about in school, so I built a scale model out of pipe cleaners and cardboard. It looks like some weirdly inspired elementary school science fair project, but it is such a useful learning tool that my graduate adviser steals it every once and a while.
Taking all of those abstract mathematical principles and turning them into concrete, real-world examples got me through my undergraduate program in Computer Engineering and helped me achieve career goals that seemed impossible to me during grade school. Now I'm working on my PhD in Computer Science through research in legged robotics. In other words, I put legs on computers for a living. That still makes me smile. My lab even looks a bit like Q's! The design and control of walking and running robots requires the sort of math and physics I never thought I'd be able to do before I learned how to learn with my hands.
Long story short: you don't have to learn new material (even theory) just by staring at it in a book! Lectures and repetitive practice drills are not the only ways to learn, and they can even discourage students who are more than capable of mastering the material through other methods. If you or your child are interested in a field that seems inaccessible, I would strongly encourage you to try a hands-on learning approach.