Is it important for young children (4 or 5 years old) in a Chinese language immersion program to be exposed to more than one type of Mandarin accent? If the child only hears one accent (and one that is not bad but not entirely standard Putonghua) for the first two years of learning Chinese, will that make it hard for the child to self-correct later when exposed to more standard accents?
I will go out on a limb and say yes, it is great to get exposure to different accents even if it is not standard so long as the child is told what the standard would be. (Awareness of the differences) The reason being that people you encounter are not always going to speak the standard pu tong hua exactly the same. So when you encounter somebody from the guangdong area for instance, you can adjust your listening to understand when a person says "si" it should be "shi". Oh really chinese has a certain rhythm to it as well. So if somebody asks you a question that is not in a set phrase you know, you can usually take a good guess at what you are being asked. (Not always but sometimes)
A related side note. Not everybody believes in the phonology acquisition cut-off age.
In Paul Pimsleur's book, How to learn a foreign language *(December 1980)*He defends the idea that Adults are better equip to learn a (second) language then kids are. And that adults can a achieve a flawless native accent. But the thing is you have to consistently work at it. So if you are talking and are misunderstood you have to immediately correct your pronunciation, and the only way to do that is to ask the person you talk to what you said wrong. And correcting the tones. Of course some people have more difficulty than others.
My little sister knew Mandarin before the age of four, but now she barely understands anything even when our family speaks Chinese, and my parents force her to go to Chinese school. So continuous exposure is important.
Even if you expose the child to multiple accents, I still think she would only be most familiar with the one that she has had the most exposure to. Personally I never learned my grandparents' dialects.
As a non-expert my answer is crude, but you'll get the idea.
If the dialect/accent the child hears contains all the phonemes that Mandarin has (from your description this is the case), the child will be able to learn Mandarin perfectly even after phonology acquisition cut-off age which is around 7. If the child learns Mandarin before cut-off age, the past language experience is far less relevant.
This question is actually broader than dialects within Chinese language. See this Linguistics SE question for more information on phonology acquisition cut-off age.