OK. Let's see if this will fly.

@@ -13,2 +13 @@
-<gloss>I wonder how they were brought up?</gloss>
-<gloss>what must their parents be like?</gloss>
+<gloss>what parents would bring up a kid like this?</gloss>

「親の顔が見てみたい」ってのは「きしょい・きもい・
うざい」とかってのと一緒なんじゃないですかね。
https://detail.chiebukuro.yahoo.co.jp/qa/q
uestion_detail/q1011633680

パワハラ上司に「親の顔が見てみたい」と言われた！ム
カつく時の解決策
https://xn--n8jtc3el8459axma.biz

I have personally heard somebody say this 
to someone in an exasperated voice (the 
way they were delivered, I'd call them 
fighting words (I did say he muttered them 
in my last comment but that wasn't really 
it)) but it does seem like that's not the 
most typical usage. But I guess "your 
mother would be ashamed" was wrong, 
because really it's a (pretty grave) 
insult directed towards the parents, not 
the child. The current glosses don't 
really convey the 悪意, though. Maybe we 
ought to make it something more akin to 
"what parents would bring up a kid like 
this"?

Does this work?

@@ -13 +13,2 @@
-<gloss>his (her) mother would be ashamed</gloss>
+<gloss>I wonder how they were brought up?</gloss>
+<gloss>what must their parents be like?</gloss>

jitsuyou: "礼儀作法や常識に欠けた相手に対してかける、このように育てた両親に会って顔を確認してやりたいという悪意のある言葉"
gg5: "親の顔が見たいものだ.　I wonder how they were brought up! ｜ What sort of parents do [can] they have, I wonder."

I think we've got the translation wrong.

I doubt it's said to the person directly.

@@ -12,3 +12,3 @@
-<s_inf>used to scold or express shock at a (younger) person's poor manners or behaviour</s_inf>
-<gloss>your (his, her) mother would be ashamed</gloss>
-<gloss g_type="lit">wanting to see their parents' faces</gloss>
+<s_inf>expressing shock at a (younger) person's poor manners or behaviour</s_inf>
+<gloss>his (her) mother would be ashamed</gloss>
+<gloss g_type="lit">I'd like to see his (her) parents' faces</gloss>