Ugh! They've gotten into the walls again!
Why would 'Six Boys Hear' be wrong here?
sé when used like that lenites the following noun and often numbers use the singular, so it would be Cloiseann sé bhuachaill
The uimhreacha pearsanta are used when counting people, so "Six boys hear" is Cloiseann seisear buachaillí.
It sounds awkward without an article.