Why did Charlemagne send a toddler (Louis the Pious) to be King of Aquitaine?

I completely understand the logic of granting out sub-kingdoms to royal heirs, to accustom them to the practice of ruling. However, I dont quite understand why that would be done at such a young age?

Its said (on wikipedia) that he was sent with a court and regents, hundreds of miles away from both parents, to rule in Aquitaine. What would that sort of life have looked like for Louis? How frequently did he see his parents? As a young king with regents, how authoritative was his word until he came of age? Was he just watching and learning from the regents or did the regents simply have a strong say in the matter?