Why are child actors typically so bad at acting?

[Chazmatazz269]

Acting is basically make believe, which is common for most children all over the world. What is it about acting that separates it from make believe, and how can directors tap into children’s imagination and creativity to get the most out of their performances?
I feel like that was the magic of “Goonies”. Richard Donner found the line between acting and make believe and blurred it for his cast. I’ve seen plenty of behind the scenes and cast reunions, and Donner repeatedly said it was a terrible experience for him. He never worked with kids to that extent again.
Shouldn’t acting be natural for kids? Is it the directors’ faults for expecting child actors to work the same as adults?

[rachel776]

the kids on Modern Family were hilarious, even, and sometimes especially the little girl Lily

[Chazmatazz269]

Lily was the worst actor by far. She really didn’t even improve with time.

[purplepoppy]

Because their first experience is doing the school nativity play where they're told to speak really loud so everyone can here. Then every kid does the exact same weird voice.

[Iron_Man]

Lol…

[Cidknee]

You're doing it with adults, cameras, lighting, and director.