Hello! I don't know if I can help, but I think that just exists 3 answers for your problem:
1- Record an Append
I don't think that the voice provider is you, but the best thing you can do in this case (When you're the voice provider and wants some effect in your voice) is record an append.
They're pretty useful in cases like this cuz you can guarantee the same quality as the original vb and get the effect. But if you aren't so it has another ways to do that.
2- Edit flags
Exists actually 3 flags who you can edit to get a childlike effect (I think):
G flag - This flags works to genderbend your utau. Positive values make an utau sound more masculine, mature and deeper, negative values can make an utau sound more feminine, thinner and childklike. This flag goes from 0 until 100, but becareful, you don't want a feminene voice so use it with caution;
D and E flags - These flags are pass filters for frequencys in mid and low end. Pass filters basically cut frequencys from your sound. If you edit your midrange frequencys I think you can get a more nasal sound and if you edit your low end I think you can cut more the deeper from the utau's voice. But becareful again, too much filter can make your sound weird (If you use too much filter in the mid range for example you will have a sound like a radio voice if I'm nit wrong).
Here is a better post about flags:
http://utau.wiki/flags
3- Mixing
Adding effects in your voice can actually help a lot, but to be honest I don't have anything to say here. You have to search for yourself. :/
I hope I could help you.
See ya~ ^-^