A LeapPad might be a good choice for such a young age. It's a lot less expensive ($100-$200) and is much more robust. It's also designed with software for little kids. They are very limited though, and not all that modern anymore either. Leo also knows of a lot of kids even as young as 2 or 3 who use iPads very effectively.
Leo also suggests looking into the Nexus 7 tablet from Google. It's a $200, small, and runs Android so it will be able to run a lot of apps. It also will be something the child could grow into over the years. Like the iPad, however, it's got a glass screen and would be more easily broken by a kid.
The nice thing about the iPad is that it has a wide variety of software for younger people, including great educational software. It would be a good way for the kids to grow into it and do more sophisticated things with it in the future, too. Not to mention it would also be a nice thing for Susan to use herself. Leo recommends the iPad with a robust, rubberized case (like Otterbox, for example) to protect it from being thrown around by the kids.