The only use for Wii Speak with Animal Crossing is to talk to your friends while you play over WiFi. If your daughter won't be playing on WiFi, don't get the bundle. If you think she will want to play online with friends (which is not necessary to enjoy the game, but is a nice addition) then I would recommend getting the bundle. It's nice to be able to talk rather than trying to type on the in-game keyboard when you want to communicate online.
(The game uses friend codes, so you do have some control over who she communicates with online if you chose to let her use WiFi.)