だって details
casual reason-opener ('cause, X); casual "even N"
「なんで食べないの？」「だって、お腹いっぱいなんだもん。」
"nan de tabenai no?" "datte, onaka ippai na n da mon."
"Why aren't you eating?" "'Cause I'm already full!" casual
そんなの、子供だって知ってるよ。
sonna no, kodomo datte shitteru yo.
Even a kid knows that. casual
Examples
'cause, X; but, X — a defensive reason or excuse offered casually in response to a complaint or question
「なんで食べないの？」「だって、お腹いっぱいなんだもん。」
"nan de tabenai no?" "datte, onaka ippai na n da mon."
"Why aren't you eating?" "'Cause I'm already full!" casual
遅刻したのは仕方ないよ、だって電車が止まってたんだから。
chikoku shita no wa shikata nai yo, datte densha ga tomatteta n da kara.
Being late couldn't be helped — the trains had stopped, after all. casual
even N; N too — a casual equivalent of でも／さえ, often defensive or emphatic
そんなの、子供だって知ってるよ。
sonna no, kodomo datte shitteru yo.
Even a kid knows that. casual
このくらいの問題なら、初心者だって解けるはずだ。
kono kurai no mondai nara, shoshinsha datte tokeru hazu da.
Even a beginner should be able to solve a problem this simple. neutral
Formation
だって、plain clause（＋もん／んだもん）attaches to plain clause{N}だってattaches to noun
Related patterns
Similar
Both open a new sentence to explain the one before it, but なぜなら is the written/formal opener that closes its reason clause on から（だ）, while だって is its casual spoken counterpart, pushing back defensively and often closing on もん／んだもん — なぜなら has no casual life, だって has no formal one.
たって is ても's own casual contraction, voicing after a nasal stem just as で does (読んだって＝読んでも), so a VERBAL だって means "even if"; the "even N" reading here is the different, NOMINAL だって — the copula's own contraction (子供だって＝子供でも) — meaning "even N; N too." Same surface, different attachment.
だって opens the turn and もの／もん closes it, which is why they so often bracket the same sentence (だって眠いんだもん) — だって announces that a rebuttal is coming, もの supplies the reason it rests on, and neither does the other's job.