Sorry for the extreme newbie question. My 10 year old son and I have started learning to play acoustic guitars. He has a 3/4 size Yamaha FG-Junior while I have a Yamaha C40. I can get very clean sounding notes from my guitar, but I'm having a difficult time getting clean notes from some strings on his, particularly the D. I'd like to try changing strings, but it's not clear whether there are "3/4 size" strings or if this uses normal steel strings.

Thanks for any advice,

any normal acoustic steel strings should work, but try a smaller gauged string, there easier to play.
their are no 3/4 size strings. just but a regular acoustic set and cut a lil bit more off the top when your dont then you would with your acoustic.
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I think that's part of the issue with the smaller guitars. I bought my daughter a small guitar awhile back. Not only did it have intonation and tuning issues, but it sounded like crap. Strings are made in standard sizes, such as 9s, 10s, 11s and 12s for most acoustic and electric guitars. Whether the guitar is 3/4 or full size, it's going to use the same strings. Unfortunately, these cheap kiddie guitars aren't the best, so he'll probably have to live with it - at least until he's old enough to start carting your guitars off to his room.
Regular acoustic strings should be fine. Classical nylon strings are easier to fret and easier to get a clean sound. Maybe try those until you and your son get a little more used to it.
While nylon strings are certainly easier to fret and don't cause finger pain, they also won't let you develop callouses. I'd strongly recommend thinking twice about learning on any nylon string instrument. It can be a crutch.