they don't like the taste of onions, they like the flavor.

I don't think the distinction between "taste" and "flavour" is the right way to frame it. Raw onion on its own can be overwhelming. If you eat a hamburger with raw onion on it, the amount of raw onion per bite will be pretty small, and it will be one taste in a whole bunch of other tastes. Your kids probably wouldn't like eating pure salt, or pure pepper either. But, food with some salt tastes great.

Having said that, fried onions are a whole different game. After 5 minutes the onion loses a lot of its potency and gets a bit sweet. After 30 minutes it's basically a very slightly pungent candy. For a French Onion Soup, you can cook them for up to 2 hours before they're ready. A pot that's full to the brim of raw onions reduces down to a thin layer at the bottom, and they taste more like gummy worms than onions at that point.

I love French Onion Soup, and occasionally make it. I'd make it more, it's just that slicing up more than a kilogram of onions is a whole process. It's so difficult it makes me cry every time I do it.