Communicating between two different "IM's" Instant messaging programs is very difficult but not impossible.
There are one or two, such as "Trillan" that will recognise all IM programs, but probably the most foolproof way is for you to use AIM, the same program as your granddaughter.
You can download AIM and it will not affect your current IM, it's a small download, easy to use and you do not need to be a member of AOL to use it.
Go to the home page of AOL and you will see instructions for the download/install of AIM, choose a user name and you will instantly be on line and on your granddaughter's screen. Lots of expressions and smileys to use. both of you will have hours of fun. Good luck, J.