Hilary Duff celebrated Labor Day with her son at the beach, and buried any ill will for her ex-husband in the sand ... all in the name of co-parenting.

Hilary and her ex, Mike Comrie, spent Monday in Malibu with their 5-year-old son, Luca. Hil was rockin' her usual 2-piece in the water while Mike chilled onshore.

Getting together for some R&R with their kid is right up the former couple's alley, post-divorce. They went on a family vacay last year to Maui ... so Labor Day teamwork seems to be a walk in the park for them.