Madonna gave her daughter one of the biggest birthday presents imaginable -- getting a packed crowd at Madison Square Garden to sing along and help her ring in the big 1-8.

The pop star brought out her kid, Mercy, onstage Monday night during her show at MSG and showed off her incredible talent by having her play a little piano for the arena -- that is, before Mama Bear helped lead a full-blown sing-a-long for her to celebrate her born day.

It was a pretty sweet moment ... not only did Madge come over and belt out the tune, but a lot of her backup dancers and stage crew members also rushed the stage to join in.

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Once they were finished with the song -- which almost everyone helped sing -- they brought out a little cupcake with Mercy for good measure ... complete with a candle and everything.

Madonna gave Mercy a hug afterward ... and it seems she stuck around to play more too.

Mercy is one of Madonna's six children -- she, along with three others, was adopted at a young age. Mercy, specifically, was born in Malawi in 2006 and came under Madge's care in '09. She's been raised by Madonna ever since ... and clearly loves her mom and then some.