Katie Holmes' Motherly Instincts Kick in on Movie Set

Katie Holmes enjoyed the hell outta mommy duty while on the job -- think, bring your daughter to work day ... but without Suri.

Katie was all smiles Friday with her on-set "daughter" in Louisiana while filming her new movie, "The Secret." She and costar Josh Lucas play a happy family of 3 and you can see Katie way into it. Hard to tell with the toddler.

Y'know what they say about working with kids.

Of course, this was probably like riding a bike for Katie ... who, of course, had plenty of practice scooping up Suri. BTW, she's 12 now!

Those sweaty gym sessions with her bae, Jamie Foxx, are really paying off based on how easily she lifted the kid during this scene shot in a park playground. Or maybe her co-star's just super light.