Brittany Mahomes is proving she's just as tough as her NFL star hubby ... pushing through a fractured back to make sure she and her fam have fun on their Mexico vacation.

The 28-year-old was spotted hittin' the beaches of Los Cabos on Thursday afternoon ... looking no worse for wear despite dealing with a painful ailment.

Brittany -- in a two-piece, green bikini -- was seen building sandcastles with her 3-year-old daughter and another child ... lugging around shovels and pales for the kiddos, ensuring they all enjoyed their day in the sun.

Britt -- who revealed Wednesday she's got a back injury that appeared to stem from her two pregnancies -- seemed to be moving around OK ... fluffing towels and playing with some water while getting dirty in the sand.

Then again, the mom of Sterling and Bronze might have had a little liquid help to manage it all -- as she was seen with an orange-colored cocktail in her hand.

Patrick, of course, was there to provide assistance too ... the Chiefs star was seen taking care of the tykes after they put a pause on their beach day.