Chinese views on Erhua
Disagree on FortCpp's answer. 

Surely Beijing dialect is not putonghua, none dialect is considered as putonghua if you put it this way. 

I'd say Beijing dialect is very close(if not the most) to putonghua. 

Back to the question, erhua is part of the putonghua standard, no question about that. For instance, "小孩" sounds (really) strange without a '儿', in fact even if '儿' is not appear in the reference, you should pronounce it unless you're from Taiwan or something :)

I can't tell you the grammar rules but cases like above are not rare.

My opinion is, avoid using erhua in general and pay attention to those cases that must have it