Translation:I am helping this child because he is small.
the "correct" answer has a typo and wanted me to have the same typo! I wrote "I am helping..." Duo wanted "Iam helping" - told me I had an extra space. LOL No way to report it, though. Limited choices for correction. I could tell them if the French sentence had an error, but not if the English sentence did...
Next time this happens could you please take a screenshot and file a bug report? This is clearly a bug as it doesn't appear in the list of translated sentences. https://support.duolingo.com/hc/en-us/articles/204728264-How-do-I-report-a-bug-
They are very similar and in many instances you can use one or the other. Parce que is a subordinating conjunction which introduces a motive or an explanation. It can also come at the beginning of a sentence. Car is a coordinating conjunction and cannot begin a sentence. It indicates the reason behind a previous statement. It is a slightly higher register of speech than parce que, and it is most often found in writing.
Both "I help that child because he is small" and "I am helping that child because he is small" are in the database of accepted translations. If it happens again, and there are no typos, could you please get a screenshot and post it here? If the system is rejecting correct answers, it's a bug, and it needs to be reported.