Just to self bump and correct myself, according to a copy of Random House and also Denshi Jisho 'dada' actually has a kanji and is not an onomatopoeic word or an ideophone.

*Onigiri = rice ball. Nikumaki onigiri is rice balls made with pork and soy sauce.
On a side note, "dada" means something like "a hissy fit", "tantrum" and is also the onomatopoeia for it in Japanese as in the sound made by a crying noisy baby, so the song is making word play with that.