I have to mirror a lot of what has been said. There's the big thing of imagining yourself in place of the protagonist suddenly getting swept into this magical, fantastical, e.t.c. world where you get to live out this epic/ideal life. It's basically a decorated version of all the early fanfiction.net self-inserts that used to be widespread (might still be there, FF has not been a frequent ground of mine for well over a decade). You and others also hit it right on the head for there being the chance for this deep self-reflection or personal growth story with the satisfaction of seeing others get to undo mistakes or pick the better choices like so many of us wish we could do thinking back on our lives. The fact that the protagonist has previous memories almost always gives them additional insight and control of what goes on around them, another desired wish of most people.
People are getting isekai'd out, which is understandable. There're a lot of copycats barely changing anything and leaning into conventional clichés just to earn easy views. Like mentioned though, so much of famous previous fiction can fall into the isekai/reincarnation genre. There's a whole lot to be explore and a lot of variety to be found. If one searches, there's probably a story out there they'll enjoy even if they think they won't because of the disinclination for the common, mainstream tropes.
I really haven't had much of a problem with the protagonist starting over as a child, but that's likely because I usually only read the more shojou-based isekai with the main character being born into some sort of noble/royal lineage where she'll either grow up fast to get right to the romance and political intrigue or else give the readers a chance to fawn over the hot dad who goes from cold to caring thanks to her adorable influence
Basically a chance for the reader to imagine getting to grow up wearing cute clothes and eating endless macarons without gaining weight
Nothing that's really problematic or icky, unlike shows like Mushoku Tensei where they have the mid-thirties MC in the body of a young child leerily watch a 14-year-old girl perform near-hentai levels of fanservice
. That was, like, 5 minutes into the second episode, and with the next scene being a minute straight of him then staring at her underwear while she was giving him his lessons, I noped out of there hard.
In regards to your (b) question about isekai'd children, that was actually a large lure when forming my own isekai story here on Tapas where, unlike most, it's going to go year by year through the reincarnated girl's life starting from birth. So much of the exploration of the story is actually how uncomfortable and awkward it is to be reincarnated with previous memories. Evianna, the MC, wishes her memories had been at least suppressed until she was a bit older, she has to deal with limitations every other isekai ignores (like babies basically being blind at birth), and unlike other stories that made some kind of joke about breastfeeding, like TBatE hosted here, she forever hates it and only does it because she faces literal starvation otherwise. I naturally don't want to say much because of spoilers, but it will ultimately lead to a question of how much of her is her and the crippling fear of her new family ever finding out the daughter and sister they think she is is nothing more than a mask she's created to fit the situation.