All news items I have read ref Android seem to point to malware being the main problem . If your granddaughter only downloads apps from google play , she should be pretty safe . I personally wont use Android , as too many apps ask or take permission for stuff they don't need . ie contact lists . Perhaps a good idea would be to get her to read the advise pages on security , as a bit of knowledge combined with some common sensce will avoid many of the pitfalls .
Best Android apps 2015/2016: Best apps for your new phone or tablet - what apps should I download?