Most of the processed foods I enjoyed as a kid (but don’t eat anymore) have already been mentioned numerous times: Hostess products, Chef Boyardee, Franco-American, all the super-sweet kid’s cereals.
I’ll add peanut-butter and marshmallow sandwiches. Had a lot of these for lunch as a kid.
This explains another childhood favorite: pancakes with sugar on top - drenched with orange juice.
Oh yeah. I had forgotten all about marshmallow fluff. I, too, at a lot of Fluffer Nutter sandwiches. But not in the last 40 years…
We always had a big jug of sweet iced tea made from the powdered mix in the fridge. A special treat on cold days was a spoonful of tea powder in a cup of hot water. Like a lot of people I lost my sweet tooth as I got older and that stuff is just ewwwwwww now.
TV dinners, pot pies, various canned pasta. Both our parents worked so we ate a lot of those during summer vacations and other days off for lunch. But now they just don’t taste very good to me.
Kind of an ersatz Crêpes Suzette. Probably better than it sounds after some adjusting (e.g. add the OJ to the batter before cooking).
Raspberry syrup - a concentrated drinks base that’s supposed to be diluted with water 1:10. Pour into a shot glass, drink the whole thing neat, sighing “aaah” afterwards and wipe your mouth on your sleeve, like adults do.
Other sugary powders: dry jello powder, dry vanilla pudding powder, dry lemonade powder, dry iced tea granules, even plain old sugar granules spooned straight into the mouth.
One thing I haven’t seen mentioned is raw cake batter or raw cookie dough. Lots of people seem to grow out of considering that a treat. Not me though, I still eat some every time I bake. It’s just better than cake that is baked, there’s a sharpness and intensity of flavour that makes the risk of salmonella worth it.
Right after my ex moved in with me (note that she’s Russian), she bought a big bottle of Smucker’s strawberry syrup from the USA and tried mixing it with tap water. “It tasted terrible!” she told me. I just laughed and told her it’s meant to be mixed with milk or poured over ice cream. (I think it’s still in the cupboard in my Moscow apartment.)
As for cookie dough and cake batter, I can’t eat them even though they’re delicious. The raw eggs in them produce a horrible itching sensation in the back of my throat that runs up into my Eustachian tubes. 
Had plenty of grilled cheese sandwiches as a kid (usually as an accompaniment to chili), and broccoli with cheese melted over it, and these were always using American cheese. I ate American cheese as recently as grad school (20+ years ago), but I would be OK with never eating it again.
Speaking of broccoli, that was always frozen broccoli when I was a kid. Also frozen cauliflower, and frozen spinach. No disrespect to my mom for doing what she had to to feed her family reasonably nutritious meals while also holding down a full-time job, but I will never eat frozen broccoli/cauliflower/spinach again. They were fine at the time, not knowing any better, but now that I have developed a taste for steamed fresh veggies, there’s no going back.
I ate lots of sugary cereals as a kid that nauseate me to even think about now - my favorites were Cap’n Crunch, Lucky Charms, and Applejacks. Although even then I hated the sweet milk that a bowl of cereal produced; my mother made sure I consumed all the milk at the bottom of the bowl, and that was a real chore.
There are other childhood favorites that don’t make me ill to think about - in my mind, they still taste good. But I bet if I were to actually eat them, I wouldn’t like them. This category includes Pop Tarts, fluffernutters, and toasted Wonderbread, made soggy by the fact it wasn’t very crunchy to begin with and then sat out with melted butter on it.
A couple of things that I ate as a kid that I would not eat now…I’d have to go with veal and escargot.
I’m a little jealous of people who outgrew their sweet tooth. I know mine is still pretty strong. I always have something sweet at the ready.
Growing up in Peru we had this stuff called “chancaca” that was literally a fist sized block of pure unrefined sugar cane. It looks like this and it is basically ambrosia of the gods. It IS just a giant rock of sugar though and eating it like I did when I was a kid would straight up kill me. Frankly the only reason I don’t eat it today is because I can’t reliably get it (apparently the pure stuff is even getting hard to find in Peru, now all you find is the processed stuff they sell at Hispanic grocery stores here). If you put a mountain of it in front of me right now I would just dig in until i died in a diabetic coma.
I’ve made something that might be the grown up version of that – waffles with a topping made from marmalade, honey, a little OJ, a splash of lemon juice, and fresh orange sections, simmered until it reaches a syrupy consistency (but with chunks of orange in it).
Smart idea.
La Choy Chow Mein was my standard “Mom and Dad are going away for the weekend, you kids have to fix your own dinner” meal when I was in high school. Loved it then. About 15 years ago I picked up a can for a camping trip - hey, it’s quick, just heat it up on the stove and eat. Never again…
Pillsbury just made all of its cookie dough safe to eat … and nestle markets a special “safe to eat” cookie dough in what looks like an ice cream container
As a kid, I sometimes ate breakfast with my Grandpa, who dipped his buttered toast in his coffee.
I copied him, dipping toast into my orange juice, which I love doing (when no one is around) to this day, 53 years later
I must say I’m surprised at both choices. I’ve loved veal all my life. I like escargot too (so long as it’s drenched in garlic butter 
), but I didn’t have it until I was 40 or so.
Where did you grow up, if I may ask?
Calgary.
I gotta ask: Have you ever had prairie oysters? 
That is a bridge too far for me.
I saved a lot of money from summer jobs and took my parents out to dinner for an anniversary. It must have been a fancy place (of sorts - could not afford a fancy place), since snails were on the menu and were ordered. I was pretty young and have no idea how common an appetizer snails were once - but it has been years since I have seen them on offer and do not much miss them.
Still, they came in a distinctive snail plate with a bunch of divots to accommodate the butter. You still see them sometimes at thrift shops.
I haven’t read much of this thread so I don’t know if they have been mentioned, but those little plastic snack packs with four crackers on one end and a blister of cheese on the other? I used to eat those with lunch pretty much every day as a kid. I’ve had some recently after years and noticed two things: they no longer include the little red plastic stick for spreading, and the contents are orange salt paste and crackers.
