You're not grasping this are you?
I certainly would have noticed as a toddler that my pasta didn't taste 'right', regardless of whether I knew exactly what was in it/on it/it had been cooked in.
I can tell the difference between brands of bread served as plain toast or even a toasted sandwich. I can tell when my partner has put too much powder in the dishwasher. I can taste the milk on the turn two days before he can.
My sense of taste was even stronger as a child, I could taste when things had been in the fridge next to something else, even once the thing had been cooked.
As an adult I can push through some of those things now... because I am an adult and I know 'its just the dishwasher powder im tasting'..
As a child, not knowing food had been altered somehow, regardless of whether that was because an adult was trying to trick me or it was just accidental,destroyed trust in my food and reduced the foods I'd eat.
As for the 'what if children didn't know beige food existed'...
Well I didn't. As I said earlier, I was weaned onto pureed versions of whatever the adults were having and my Mother cooked from scratch and was a good cook.
The 'beigest' foods available would have been pasta, brown bread, rice or potatoes, there was never sugar in the house, she very rarely ever made cakes, there might occasionally be a digestive biscuit or a rich tea but never anything more extravagant than that...
That didn't change until we were 10 or 11, when my Dad hit the roof and started a massive and memorable argument about it as he did want the odd dessert/biscuit etc (this would also tie in with him retiring and not having access to the tea and biscuit trolly at work where such things were supplied!!)
My food refusals/avoidance were not 'I want something ELSE' they were 'I can't eat this but I am still painfully hungry* and that remains the case.
I 'same food' a lot - sometimes thats not the most healthy thing (quorn dippers, interesting I never ate chicken dippers as a child and hate the texture of chicken!) - sometimes it is though, for months through summer I had a finely chopped salad every day, until as I say, ingredients got more expensive/went out of seasona nd things like apples started to be floury and bland.
ARFID is certainly not about refusing to eat certain foods in order to be offered other, nicer, sweeter, fattier foods.
In reply to someone up thread - it is very limiting and can be stressful - I am currently worrying about a work event at the end of bloody June, where I will meet my work colleagues for the first time and have to eat in front of them. I have already checked the hotel menus and think it'll be ok but I really won't know until the day and will continue to be anxious about this until its done!
However compared to my life as a child... it is a million times easier. I can attempt to voice why I can't eat certain things, what is wrong with them, why this McDonalds fry is acceptable but that pile there are all duds...and can figure out what might be acceptable, what I can try safely... and I can do all this without pressure or being mocked or abused in my own home.
Children just can't do that, they haven't the life experience or vocabulary to do that, they don't have the control over what comes into the house, how things are cooked, they don't have the knowledge to make suggestions.
And sadly many are mocked, bullied and abused by those around them who should be supporting them.