the directory will be in your C: drive (or what ever letter your hard drive is), it is where the programs files and programs x64/86 is. once there open the users folder and chose yours(not default or public, usually the name you use or the user profile you set up for your daughter, if it's a shared computer). in your user folder open appdata and then roaming. .minecraft should be there at the top. click it and then the open button in the forge installer(assuming you are using the forge installer), and it should finish installing with the correct minecraft directory. run the game with the new forge profile on the minecraft launcher once to the main menu(don't need to start a world). then quit and go back to your .minecraft folder(pin to quick access to save time) and there will now be a folder called mods, drop your .jar mods here. then reload the game with forge profile and enjoy. I use a lot of mods(http://modlister.com/user/314kojioyama) and am not great at debugging things, but if you need further help or more instructions ill try to help and explain best I can.
PS the logs folder in .minecraft is the place to go if someone asks you for a log to help with crashes or other bugs.