tl:dr "ihminen" is in this case sounds unnatural and is grammatically wrong.
In this case "ihminen" wouldn't work (except if you add either " , " or " ja [=and]" between them) since both "ihminen" and "lapsi" are nouns, and "hiljainen" in as adjective. You can't put 2 nouns after each other since it sounds unnatural, so you need to use an adjective when describing a noun or another adjective.
Crash course on how to spot adjectives in finnish: if the word ends in -nen, it is most likely an adjective (hiljai-nen (quiet), kultainen (golden), likai-nen (dirty). (except some surenames like "Pöllönen" in the early courses).
Also if you want to say "Hilla is a quiet child", you would say "Hilla on ihmislapsi", but not gonna explain why it is like that since even me, native, had to use dictionary to search that up.
"Hilla is a human child" - "Hilla on ihmislapsi" sounds a bit strange, but that could of course be possible. However, if you do "put two nouns after each other", you form a compound word in Finnish, like with "ihmislapsi" (ihminen+lapsi).
suklaa + kakku = suklaakakku
mansikka + pirtelö = mansikkapirtelö