When little kids go to school for the first time, their language gets a step change in clarity.

When kids are learning to talk and they're just talking to their parents their parents understand them even when they're unintelligible to others because the parents remember what that sound signifies from direct prior experience.

The parents are reacting less to what the utterance sounds like and more recognizing it from what they know it's connected to.

If you say a thing and it's unintelligible to the receiver, you have to modulate and try again.

If the receiver understands it, there's no need to modulate, because it achieved its goal.

But being with a bunch of peers, if they can't understand you you need to change how you speak.

It's only by failing to be understood that the child has an incentive to produce the utterance more clearly.

English is an equilibrium of being mutually intelligible.

So kids at school have much better language production.

They have to be intelligible to a lot of other people who don't have the time or mental capacity to remember every random person's specific speaking style.