You don't say whether you are stuck with an existing recording that you are trying to alter or if you are starting from scratch. But, from scratch, here's what I have done. I don't claim that you will mistake it for a real child, but, it definitely doesn't have to sound munchkin like.

I used to do this with a pair of reel to reel recorders, but, Vegas would make it even simplier.

What I did on my R2R's was to record at the slowest available speed. I would speak in a normal tone, but veeeeeeerrrrryyyyyyy ssssssllllloooooowwwwwlllyyyy, taking care to control also the rate of my inflection (keep it slow - ie vary the pitch of your voice from low to high to low very slowly. Then, I would play that recording back at a higher speed and capture it to the second recorder.

I haven't played around with this in Vegas (have had no recent need for this effect), but, imagine you could accomplish the same thing by simply recording yourself in Vegas speaking very slowly as described above. Then, alter the pitch.

Actually, on second thought, I bet you could simply speak normally and just alter the pitch while maintaining the orignal duration. Yep, that should do it. Use the Sony Pitch Shift plugin. Experiment with raising the pitch and check the box to "preserve duration."

Have fun and let us know how it works out for you.

Del