Hi Audacitizen !
I would like to register my wife’s voice and to change it in order that it sounds like a 6-8 yo girl.
Do you have any tips or ideas of want i can do with Audacity to help me i doing this ?
The best way would be find an actual kid to do it. 
Or, maybe your wife (or someone else) can mimic a little girl’s voice.
Just about every school classroom has a class clown (usually a boy) that can “do voices” so there’s not shortage of people who could do it… 
It’s easier to mimic a voice if you have a reference sample to listen to, so find a recording of an example-voice like you want to mimic.
You can try the Change Pitch effect but it probably won’t be realistic sounding.
Effect > Change Pitch sounds like it should be perfect on the tin, but in actual use it changes everything about the voice and everything doesn’t change between different people. For example, if a very large man and a small child both whispered to you, you might have a very hard time telling who was who. The the instant they started speaking different parts of their voice kick in and it’s obvious.
So, not, this is not a simple job.
Someone posted a voice changer technique and I’m looking now.
Koz
Here it is. This is just fooling around, but you might find something useful.
https://www.youtube.com/watch?v=ztNBCEq8rxQ&feature=youtu.be&t=2m30s
Koz
The easiest way is to change the pitch of the voice. Go to Effect > Pitch and Tempo > Change Pitch, increase by 4-8 semitones. If you find the voice too fast, go to Effect > Pitch and Tempo > Change Tempo, reduce slightly.