Childish, yes, my book seems to imply that with this example sentence: ねえ、お母さん、あのおもちゃ買ってよ。僕どうしても欲しいんだもの。It's defined as 'expressing dissatisfaction or desire' at the end of a sentence.
So, the difference in tone...
刺身注文しないよ。食べられませんから。"I'm not ordering sashimi because I can't eat it."
刺身が欲しくないよ、大嫌いもの! "I don't want sashimi, because I hate it!"
Do these examples give the tone you're talking about? Were I to say 'taberaremasen mono de', would it sound like an apology, 'I'm so sorry that I can't eat it'? Sorry to hit you with so many small things at once.