Answer for iPod for a 9 year old

There are a few different iPod models, but most likely you are asking about the iPod Touch, since the other iPods are primarily only music-playing devices. There is no monthly fee for an iPod touch because it is not a cellular device. It only can connect using Wi-Fi. Therefore there are no fees to pay to cellular companies.
The only real difference between iPod Touch models is their storage capacity. They are all the same in terms of computing power and capability. Color is, of course, a completely aesthetic decision. Regardless of the age of the user, the 16 GB model ($199) will only suffice for people who download few apps and don’t take a lot of pictures and video. For only $50 more, the 32 GB model is double the capacity and most average users will fit comfortably in that amount of space. The 64 GB model is $100 more than the 16 GB model and the quadrupled storage is definitely an accommodating amount even for users who download a lot of apps and take a lot of photos and video. There are probably some users out there that would be willing to pay $399 for a 128 GB model, but I’m guessing that isn’t something you’re considering for a 9 year old child.
Knowing the way most kids use their devices, they tend to eat up storage quickly. Personally, I would shoot for the 64 GB model ($299) and only drop down to the 32 GB model ($249) if your budget simply won’t allow it. But consider that if you spend a little more money now for the extra storage, you may not be pestered in a year or so to buy a new iPod because their current one can’t handle all their apps, photos, and videos.