This is the tough part. The target audience is a little mixed as we do have two agendas.
We need to appeal to the children's parents and build trust with them as they will be the ones signing up to the system. We also need to appeal to the children as they are the day to day users of the system.
Important: The biggest mistake would be appealing to children of too young an age group. We are NOT appealing to kids younger than 8 years old, so little kids stuff won't cut it. Think tweens through to 16 year olds and you are on the money.
Go too heavy on the kid front you risk not gaining trust.
Go too young and people think it isn't for their kids
Go too heavy on the trust and security front and you risk the kids hating it.
Some how Google manages this concept of mixed agendas. Fun, clean, simple, modern, transcends ages ... and at the same time it seems reliable, trustworthy and has an honesty about it. Apple manages the same thing on their sites/products/packaging (but perhaps a little more sterile).