Bukkit plugins require running a separate server and giving the kids access to the server. Doesn't seem as practical as a client mod if you're trying to make it kid friendly.
I agree a client mod is a great place to have this functionality, but if it is being done on the server as well then using a framework like Bukkit is a great choice.
For one thing, the author complained about obfuscation making it difficult to expose different functions in the game. Bukkit provides a stable API, so it makes sense for a scripting wrapper to expose those functions, and by doing so gain a lot of functionality.