Pepper coloring aside, it seems your child is suffering from the common human foible of doubling down when presented with contrary evidence. You might gently talk them through guarding against this trait in the future? Just a thought.
Red (or gold/orange/yellow if full ripe) ripe peppers, whether sweet or hot, share multiple reasons for different pricing:
One, they’ve got a more complex flavor and higher nutritional value. (Picking them green will maximize productivity, see below; but it maximizes quality only if your standard of quality for that particular type of pepper includes that you prefer the green ones, and/or includes long storage time in the fresh state after harvest.)
Two, the skin does soften as the pepper finishes ripening; they therefore are more liable to injury in the field, to injury during harvest and transport, and to rots anywhere along the line; plus storage life is shorter (though they can be dried or frozen.) So handling becomes more complicated and losses both on and after the farm will be greater.
Three, the plant puts more energy into each individual fruit if they’re left to ripen, and will therefore probably produce fewer individual fruit. (If the whole field is harvested at one picking, this isn’t an issue; but if multiple harvests are taken from the same plants, then it is.)
Four, in some markets they’re rarer and may be priced higher due to scarcity; and in some markets they may be considered more desirable, whether for reason one above or just due to what’s in style, so may be priced higher for that reason.
More than that, said child is suffering from the “truth through consensus” meme – if enough people believe it, it has to be true.
In fact, here’s a picture of them at the grocery today. “Grown in Mexico: Red Jalapeno Peppers $1.49/lb.” (The greens were at $0.99/lb today, so, percentage-wise, these are significantly more expensive, but still pretty inexpensive. They go down as low as $0.99/lb for the reds when they get a glut of them.)
I’ve had a backyard garden for most of my life and in my experience almost all peppers will rot on the plant before they ripen into red or whatever other color. That’s why green is the most common variety. It takes just the right kind of soil, moisture, fertilizer, care, singing nursery rhymes to them at night, etc. to get them to fully ripen into color. Of course, that is just in my climate.
But I know that if I had to sell them, I would want a damn sight more than a 50 cent per pound markup for them.
Not at all my experience, and my garden is pretty much half tomatoes and half peppers. I’ve been growing them for at least 15 years now. The vast majority of my peppers are picked red and then dried for use throughout the fall and winter. (I already have red peppers in the garden, and I live in Chicago, so not a zone that is warm year round.) (I don’t do anything special with them either. I don’t pamper them. I haven’t even watered them once this year. They are in a location that only gets direct sun after about noon.) They do fine, despite my rampant neglect once they’re in the soil.
Are the red ones supposed to be hotter? I hadn’t noticed that. I have noticed that jalapenos with leathery striations along them, are hotter than those without. Smaller also seem to be hotter than large ones. We have some for sale here that give pasilla peppers a run for their money in size, and those are quite bland.
Bullshit. You’re just a lousy gardener. 
I usually grow between 5 and 8 different peppers each season, and almost all of them will go red if allowed to. The poblanos just get a darker purple/black, but the Anaheim, jalapeno, banana, serrano, yellow and such all go red. Ditto the bells - Yolo Wonder, etc.
The only hot pepper that doesn’t are my habs, but that’s because I grow a particular strain of them that max at Dayglo orange.
In my experience, the color doesn’t have much to do with the heat level. If it does, I don’t notice it. The main difference in flavor is the sweetness. Think red bell pepper vs green bell pepper. Green chiles taste grassier; red ones taste fruitier. One isn’t better than another. They are different. (Though, for drying, I want them red. When I cook, I tend to use green ones a bit more often.)
What varieties are you growing?
Some are bred to stay green as long as possible, for green stage harvest. Others the reverse. I ripen peppers in upstate New York and others do it further north than that – I’ve bought seed from Maine and from Montana.
That’s generally caused by direct sunlight exposure; some varieties of jalapeño are more susceptible to it than others. As amount of sun during the season also affects heat – more sun, more heat – there may indeed be a correlation, within a particular variety, with heat.
Do you mean from the same plant? Or do you mean as found on the market?
If the latter, it may be different varieties – some run hotter than others, and some run larger than others.
peppers are Central/South American in origin. if I can get good ones up in Michigan, you can too. you just might need to fertilize appropriately.
I have a similar problem with tomatoes. blossom end rot is kicking my ass on one of my plants, and as near as I can tell it’s because the ratio of calcium to nitrogen is out of whack.
A soil test is a good idea, for both of you. And excess nitrogen can be as much of a problem as not enough of it.
But blossom end rot in tomatoes is very often a water supply problem. If there’s not a good enough consistent water supply, then the calcium won’t get all the way out to the end of the fruit, no matter how much calcium is in the soil.
Yup, this^. I had a problem with blossom end rot on my tomatoes this year. I installed a drip irrigation system on a timer and, voila!, no more end rot.
Until this thread, I had no idea. I suppose I never sat down and thought about it, but the only ones I’ve ever eaten have been green.
I knew that jalapenos turned red when they ripen. Like many other Dopers who’ve posted to this thread, I’ve seen them turning red in the bins at grocery stores, farmers’ markets, etc.