I'm using a Reaper VST plugin for this in an attempt to modify an adult voice to sound like a child's. Besides pitch, what else can I modify? Changing pitch just makes you sound like a chipmunk.
Voices are complex things and, as you have noticed, playing with pitch or speed makes things sound weird.
While pitch is the most obvious characteristic, there are two other things to take into account.
One is the formant - Formants can be thought of as the basic waveforms of a voice - If the formant is of an adult male voice then pitching it up will simply sound like and adult male voice pitched up. If you shift the formants up without shifting the pitch then you'll end up with something that sounds like a female or child's voice that has been pitched down. The formants give you the characteristics that you recognise as male, female, or child regardless of pitch.
There are formant shifting VSTs out there - I haven't used one for a very long time but years ago I used Steinberg's Voice Generator plugin.
The second thing to consider (and has already been mentioned) is performance. If you have the pitch and formants set correctly then if it still doesn't sound right then it because the things being said and the way they're being said are at odds with the sound of the voice. Just as you wouldn't expect an adult male voice to use baby words you also wouldn't expect long, complex words from a baby voice.
Obviously, the voice acting is the most important part, especially given that there are adult actors who voice child characters with no electronic processing at all. But if you want some processing to expand the sonic reach, you'll need independent control over pitch and formant to get a convincing result. Control over just pitch is not going to give it to you.