I never did the child avatar thing, but I wanted too when I first started.... however I didn't quite understand resizing and I'm a furry, I can't have a giant head on a child body. Doesn't look right.
That being said, I have furry avatars of all heights. One of them is a Kani (rabbit avatar, which cracks me up because Kani means crab in Japanese...) and I was told before that the default body was 'too childish' and the person gave me a 'mature' body for it... and then the Kani head was too small! IT WAS CREEPY.
And then I met a 'baby fur'... the RL person was an older individual that had to be cared for... and it showed in their avatar.... It was cute for the first... oh.... 10 seconds, then it's diaper scripted a poo action and it made baby noises. Not cool at a club, just saying. I'm there to dance and have fun, not care for a baby fur.
At the same time, going back to my Kani avatar and the hybrid I made using the head... Yea, I can look like a teenager or young-young adult, and I haven't really recieved negative feedback, sans for being a fur. I see 'anime' or 'kawaii' human avatars and think 'huh, you look 16' and move on with my life. Young avatars don't bother me until they do the iccle speak, or have annoying scripts that make them soil a diaper and then whine about it until someone clicks on the menu to change it.
Now, being 25 irl, there are times I don't want responsibility, I want to be a teenager. At that point I don't go to adult sims where my RLV can kick in and trap me in something too mature for that age group. I don't talk like a toodler. I don't act disrespectful (unless it's a bot or AFK avatar, then let's face it, it's fun). I also have a GIR avatar I will use to cause random havoc... but once again I stay away from adult sims where role-playing would not appreciate the avatar of my choice.
I think that's the downfall of young (child and teenage) avatars: the lower level of respect. Often (not always) the RL person doesn't care to respect the whims of the sim they are on, it's all about them, which is a child mentality. The amount that occurs yields to a general and bias unlike towards the avatars.
If I see a child avatar, I will treat it like any other avatar, and I don't have to stick around to mother it if that is what the RL person wants.