I also think it is a great idea:D I have 3 kids and they are always asking if they have mail. Even when they get there check from our business I never know if they are more excited about the money or just getting mail. I tell them all the time if they want mail then they should write to someone so they can write back.
With this idea they would know that they will get something just for themselves in the mail...I think it is wonderful (as long as the price is doable). What would be a fee you are thinking of (or have you even gotten that far?) How about allowing the kids to write back to the character asking questions or suggesting places to go next? Great time to start it too since you can promote it for Christmas gifts...their first one being in December.:D