Genie, yes "allègre" means indeed what you say, but the accented "è" is stressed and open, the final "e" doesn't sound, and there's no way that it could be stressed and both "è" and "e" be pronounced "é" because it wouldn't be understandable in French. I agree that songs sometimes do play fast and loose with normal grammar and twist prononciation a little but even loose grammar and twisted pronunciation need to be understandable. It's as if I pronounced some English word sounding like /stayPLAY/ (don't read it again but just say it over and over...) you'll probably wonder for ages what I meant (it's the next quizz, ah!)
It can't be "ton/ta/tes" (possessive determiners) "danser" (verb); even in the very unlikely case where "danser" would be used as nominalized verb, it'd be "ton", not "ta".
Is there any sheet music or a record sleeve or any document with the printed lyrics?