I recall earlier hinting, but it is very infrequent and feels like it's used to justify their relationship ("Hey, I had a crush on this person when I was younger, and I need to find them because they're still on my mind even though I'm with you"/"Hey, that person was me! Now we can continue our relationship with no worries.")
Not a childhood crush/friends thing, but I was reading this other BL that did something similar. We're about 50 chapters in, and the ML randomly mentions that he and MC met once before--not as kids, but shortly before the series started. It has no effect on the narrative and he doesn't even tell MC, so...why waste a chapter?
nooo.. just to make the relationship feel more "destined" without actually putting much effort into it..... it always takes me out of the story (˃ᯅ˂) you'd think KR authors would drop dead if they couldn't find some way to make the leads' childhoods intersect lmao. i've definitely had this exact same convo with a friend before, can't remember the titles... lowkey felt it with 'while you weren't watching' too
tbf i love childhood friends to lovers when it's done properly and actually integral to the plot (ㅅ´ ˘ `)♡

I say this so often with BLs I read, but it's true. Like, the couple already had their own story going on. Why did the author bother giving a "they met as kids and had crushes on each other" backstory? It's not important--like, at all. Heck, his mom only briefly feels guilt for forcing ML to be a girl as a kid and moves past all her stress about him being into guys because she had something else going on and said they'd talk later.