The retroflex r is not the consonant r in a word (pinyin) like in 人 rén.
The retroflex is added as a suffix in the Beijing area, or as a diminutive suffix.
Would it be pronounced with the second tone, because 儿 is pronounced ér?
Or would it be pronounced like the previous syllable? Like in 哪儿 it would be pronounced 3rd tone, or 4th tone as in 那儿?
Is there a rule to pronounce that retroflex?