Alternative voices

I’m working on an app where I need kids voice (or better voices), and the current selection doesn’t give anything close to a child’s voice except for Ivy voice in Alexa.

Is there a way to use more voices, other than using some third party to generate them and then use as a stream?

I just really want to make the app experience more like “my best friend Tom” and leas like a “lady from the echo”.

I don’t think so. Everything you can do via speech steps (SSML) is pretty much on this documentation page:

FYI: Amazon Polly is a way to test SSML tags without uploading your skill: