Do child actors ruin otherwise good movies?

Maybe in this era but not in the hey days of Hollywood into the 60's. Shirley Temple and Peggy Ann Garner are exemplary  examples. As well as all three child actors in the film Houseboat 1958 and countless others. You need a different era of films, that's why I'm a Turner Classic Movies junkie. Give me a B&W from the 30's, 40's, 50's any day. When all the actors could act, young and old.

The kid who played young Anakin Skywalker in the star wars series was brutal. Very cringeworthy when Natalie Portman was leering at him like a pedophile first time they met and then banged him later.

No, sometimesw they make the movie better.

In the case of Terminator 2 I agree. Edward Furlong's high pitched screaming like a girl ruined that movie for me.

Sometimes.  But if that happens, you can't blame the child actor.

The fault lies with the casting director, the director or the producer.

They've either cast a kid with no or little talent, chosen a child for their looks or who their parents are, or have failed to give good direction.

Sometimes ... but I often wonder if it's the actors or whether it's the script and direction!

I always found Spielberg's War of the Worlds annoying because of Dakota Fanning. She's really good but she was obviously told to deliver the lines like a sassy college student ... but at age 10. 

Then there's the 'bowler kids' ... the never ending procession of boys with haircuts nobody's had since the 80s who have ... to ... enunciate ... each ... word 'cause the director worries that the audience won't hear what they're saying, particularly since Americans clearly love kids with slight lisps too.

Stranger Things (certainly 1st season) is a good example of a young cast who aren't annoying. They actually speak like real kids ... they talk over one another ... they mumble ... they poke fun at one another ... they swear when parents aren't about. 

It can't be the case that they're the only good young actors so it must be poor scripts that really annoys us.

If they can't act, yes.  If they are good actors, no, not necessarily.