noun, noun or participle which takes 'suru'
Sentence-ending particle used in casual speech, often by women or children, to give a reason with a tone of mild protest, insistence, or emotional emphasis. Similar to のだもの or んだもの.
だって、お腹すいたんだもの。
But I'm hungry, you know.
まだ子どもだもの
After all, he's still a child, so it's natural he doesn't understand.