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We don't see too much of the Irwin kids , but this weekend Robert "Bob" Irwin turned 9 -- and he's gotten so big!

Hard to believe it's been over 6 years since his father, Crocodile Hunter Steve Irwin, was tragically killed by a stingray in 2006 ... before Bob turned 3.

The family is keeping his legacy alive at the Australia Zoo, AKA "Home of the Crocodile Hunter," where they celebrated Bob's birthday and still make regular public appearances.

Both Bindi -- who's now 14 -- and Steve's widow Terri were on hand for the event as well, posing for photos with Bob and his birthday cake.

Great to see these kids so happy and healthy. See how they've grown through the years in the gallery above.

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