It can't. The subject "it" (in Polish it is inferred) is neuter here so you can say either "będzie padało" or "będzie padać".

PS I highly discourage using Google Translate. There are a loooot of much better tools and dictionaries you can use for any specific purpose or generally as a substitute.

it is because "it will be raining" can be also translated as "będzie padać/padał deszcz". Default duolingo translation does not specify if it will be raining or snowing.

basically , there is no change in meaning between padać and padał/padało, but if you use past form, it has to match with subject.

We generally don't accept diminutives unless it's really justifiable (like when the original sentence has "little girl"). Apart from that, that would create a normal sentence, just less formal because of the use of diminutive.