I’m gonna suggest the little-known Nicholas Hoult for best child actor performance in the equally little-known movie About a Boy. Nicholas is the boy, but Hugh Grant is another adult boy who plays a completely self-absorbed trust fund baby who’s never worked a day in his life. In the process of screwing his way through every single mom in town, he encounters Nicholas and his mom, and Nicholas will not let him get away with his bullshit. It’s really a remarkable performance, and IMHO the best ever by Grant, though like some other roles here, doesn’t seem to be a stretch at all.
Miko Hughes wasn’t great in Pet Sematary, but to be fair he was only about 3 years old. I like that movie a lot, but I wish they would have used a slightly older boy who was a better actor and didn’t look directly into the camera.
Neil Patrick Harris would like to have a word with you.
I remember Jodie Foster as being a great actor as a child, but maybe my memory is clouded because of her success as an adult.
another vote for King Joffrey.
the kid with the smirk in every scene of Once Upon a Time is rather bad.
The girl who played the eldest daughter on the tv show Medium, Sofia Vassilieva, is an amazing actress IMO. There was at least one episode where her character is prominent and I’ve also seen her in other roles outside of Medium and she’s always remarkable to me.
The Greenbush twins (who played Carrie on Little House on the Prairie) were dreadful. They couldn’t do much except fall over. I think the show recognized their limited capabilities- they had to bring in new unrelated kids to keep the stories going as they got older.
I agree but he’s hardly a child actor. Jack Gleeson was 18 when principle photography began. Maisie Williams (Arya) was 13 and Sophie Turner (Sansa) was 14.
Ariel Winter is tremendous.
It was on METV recently, and of course the voice was easily recognizable (to me) as that of June Foray (who also did Talking Tina, the evil doll).
It was also the very last show of the series.
Also, the name of Mary’s character is “Sport”, and the boy’s name is “Jeb”!
Not to mention Rocky the Flying Squirrel, and most (if not all) of the female voices in the old Rocky and Bullwinkle cartoons.
I remember the Dondi comic strip, though I don’t recall ever reading it; the movie was slammed in Medvedev and Dreyfus’s The Fifty Worst Films of All Time, with the kid being named the worst child star ever, IIRC.
Some of thse replies have stretched the age requirement for child actor, so perhqps i should set a limit. I’m thinking the 13 and below crowd (most likely the pre-pubescent crowd) as a good age group, but use your own judgement.
Ok, my second good and bad nominations
Good (well great, IMO) would be Tatum O’Neal, who I did not see mentioned in the thread. Apologies if this is a duplicate. Her acting in Paper Moon was Oscar worthy. I also second or third Haley Mills. The original Parent Trap was when i first saw her, and she was great in it. And that movie has held up well, primarily because of her acting.
The worst goes to a three headed monster known as the two youngest children on the Partridge family.
Suzanne Crough as Tracy Partridge -truly awful. It’s hard to pick on children so young, but she never improved during the shows entire run. I saw her interviewed on one of those “where are they now” type shows, and she was as bad an interview as she was an “actress”.
Brian Foster as Chris Partridge, who replaced the god-awful
Jeremy Gelbwaks, the first Chris after the first season. My favorite note from IMDB says that Jeremy was replaced after his father got a job that moved the family out of California. ABC did not receive one letter of complaint about the change. 
He’s not that bad, but someone needs to tell him to stop smirking.
Conversely, Bailee Madison, the young actress who played a young Snow White in an episode or two was wonderful. She absolutely nailed the older actress’s mannerisms and facial expressions.
Looking at her IMDB page, she’s got quite an impressive resume already.
And she really kicked ass in the movie “Hanna”. Just a great actress.
I thought of a bad one: that kid who was in the Terminator movie.
Jodie Foster in The Little Girl Who Lives Down The Lane gave me nightmares. Best Performance Ever.
I just read Patty Duke’s autobiography, and she says the slapping in the dining room scene of the Miracle Worker was quite real. She said how difficult it was to not tense up when you knew it was coming and there was a time she had her jaw clenched and ended up getting part of a tooth chipped off! There were 719 performances of the play, so that meant she got slapped over 1400 times. Interesting read.
The twins that played Carrie Ingalls on Little House on the Prairie were so bad, the Ingalls had to adopt the totally fictional Albert, James, and Cassandra just so the scripts could push non-fictional Carrie into the background where no one could find her.
Bad: Jerry Mathers
Good: Patty Duke
Tatum O’Neal:
Nobody loves Paper Moon any more than I do. I’ve seen it dozens of times, one of my very favorite films. However, everything I’ve read or heard about the making of this movie strongly suggests that director Bogdanovich should have gotten the Oscar for painstakingly walking a wooden non-actor child step-by-step through what ended up looking like good performance. Directing in the most literal sense. Her subsequent roles support this notion, IMO.
Not that I care- it’s a perfect little movie, and however it was accomplished, Tatum was wonderful onscreen. As was her father- PM is IMO the only good performance (great, actually) of his career.
Only opinions- I reserve the right to be wrong.
Jeff Bridges, not Daniels. But an excellent recent example.