I want to do 2 more returns for my daughter and granddaughter what package do I need

You can simply create 2 new returns for each of them. To do so click on the tools menu, and choose return manager. From there you can create and switch between the various returns.

Is there a limit on the number of returns you can file under the same login? Is there an additional cost?

The CRA limits the number of returns under a single login to 20. Each additional return will have an additional charge depending on which version is used. The cost will be shown when you create a new return.