I think there's some misunderstanding about how children learn. So you start with a young child, say 18 months. She doesn't understand how she's meant to behave really. She hasn't got the maturity to do that. She snatches a car from her big sister. Obviously there's infinite ways to deal with this, but here's two.
Route A: Give car back to sister, saying, "No, don't snatch." Toddler is upset. Screams blue murder, snatches car again. Car is given back to sister, "No," is repeated. Toddler is removed from situation (could be different room, or play pen). Toddler has tantrum, then gets over it.
Route B: Say to toddler, "That's not very nice, dear, look your sister is upset. Do you want to give it back to her? No? How about you have this car instead? No? Oh dear, shall we have this one? Look, we can roll this one down a slope? Isn't that fun?" until either the toddler drops the car or the sister gives up. (Though if you've gently parented the sister, she might take more direct action herself to be fair!)
Route A seems unappealing. You end up with an upset toddler who doesn't understand she's done something unkind. However, she's done something unkind and had a negative experience as a result.
Route B however teaches her that if she snatches toys she'll get lots of parental attention and there's no downside.
I'm not suggesting that toddlers reason this through and are deliberately manipulative. But they're also not stupid, and they learn all the time. If an action gets a toy they want plus lots of parental attention, then surely that's worth repeating. In other words, people who don't take action because their children are too young to understand they shouldn't do something might be delaying their children's ability to understand that.
I'm aware that both routes I've suggested seem to fall within "gentle parenting" as defined by different people on here. But my experience of people who call themselves gentle parents is that they'd tend to go down Route B.