I was not allowed to wear flip-flops as a kid. My parents had old-fashioned ideas about kids having to wear solid, heavy Oxford shoes at all times lest their ankles or feet become crooked, or something like that. I didn't have any orthopedic issues or injuries, either.

I don’t know how common the heavy shoes idea was (I’ve never read of anyone else coping with wearing socks and heavy shoes all summer, on parental principle) but both my parents bought it unquestioningly.

The only summer I was able to circumvent this was after grade 2, the year I made my first communion, when I took to wearing the white shoes from the quasi-bridal communion outfit. They were light and slipped on and off easily, so convenient, and I wore them until they fell apart. But they must have looked odd along with the tomboyish shorts and t-shirts I tended to wear.

The other footwear fight I had with my parents was an absolute proscription against being barefoot. If they spotted me slipping to the bathroom from my bedroom without slippers on, they would hit the roof. Was this because of some association between bare feet and beatniks, as they persisted in calling counterculture people well into the 1970s? I don't know what other reason they might have had, and even if I could ask them now, they would deny it had ever happened.

Editing to add: Google AI replied, when I asked about why it was believed that children should wear heavy, solid footwear:

Historically, it was widely believed that children should wear solid, rigid lace-up shoes at all times because parents and medical professionals thought a child’s developing feet and ankles required external structural support to grow straight and strong. From the late 19th century through the mid-to-late 20th century, letting a child walk barefoot or in flimsy shoes was thought to risk permanent physical damage.

There was more, but this is the gist. It also adds:

Modern pediatric podiatry has completely overturned this consensus.

Vive les gougounes!