Is this valid syntax in English? 'The man who is afraid', yes, but I have to say your suggestion sounds unnatural to my ears. Interestingly, if you search Google using quotation marks, "the afraid man" returns only 842 results. Similarly the numbers for "afraid child/person" are very low compared with the adjective following the verb, and I think indicative of the syntax irregularity. I also found these threads which you might find interesting to look at: https://english.stackexchange.com/questions/69178/is-the-phrase-the-alive-animal-grammatically-correct
I don't quite know why I wrote this, it certainly isn't what I would normally use, it slipped out without thinking, and I agree it does feel awkward. However, I wasn't aware that it was actually grammatically incorrect as you link indicates. I guess it is something that might get used occasionally in practice, but evidently it is incorrect usage. Interesting!