Jennifer Lopez and Ben Affleck aren't getting "On The Floor" together anymore ... but, she's clearly a committed ex-stepmom -- 'cause she took his son to lunch!

The singer-songwriter took her child Emme to lunch on Saturday ... and, she stunned in a white crop top which showed off her ripped abs. Gallery Launch Gallery Backgrid

She added a pair of low-rise black pants to the ensemble as she walked with Emme and Samuel , Ben's 13-year-old son.

Check out the pics ... Samuel looks a little like his dad in a t-shirt and some pale shorts as he follows behind J Lo. Gallery Launch Gallery Getty

No Ben on this outing ... just Samuel hanging out with his former stepmom and stepsibling -- which makes sense when you consider how close the families are when they were link

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