Looking for a young adult book from the 80s about a kid who has a robot "sibling" who crashes(?) on a planet full of child robots. The title had some sort of acronym in it.
This sounds like C.O.L.A.R. a children's book by Alfred Slote published in 1981. It's a sequal to My Robot Buddy, where Jack Jameson, an only child, convinces his parents to buy him a robot brother named Danny for his 10th birthday.
In C.O.L.A.R, the Jameson family is returning from vacation on M Colony when they run out of fuel and have to land on an empty asteroid. It turns out that the asteroid is actually populated by runaway robots ("mostly boys and girls, since they cost less to manufacture than adults."), and that they call the planet C.O.L.A.R - "Colony of Lost Atkin's Robots".
There's a third book in the series, called Omega Station, which continues the adventures of Jack and Danny.
From the back cover of C.O.L.A.R:
Summer vacation on M Colony had been one of my best with Mom and Dad and, of course, Danny my robot buddy. But didn't it just figure that on our return to Earth we ran out of fuel and had to land on some desolate planet? No tree, no grass, no water, no colors. Nothing but gray craters and gray rocks. It sure didn't look too friendly.
Suddenly, five huge rocks started moving towards our ship. They were making a shrill and deafening noise and it looked like they were out to destroy us! But good old Danny ran outside and rescued us in the nick of time. But then - and I know this sounds unbelievable - the rocks just sank into the ground and Danny was nowhere in sight. Where the heck did he go? And what did those rocks have to do with his disappearance?
I had to find Danny. True, he was all silicon chips and metal where I was flesh and blood, but Danny was my friend - and brother. It was up to me to save him, no matter what.