Tristan and Khloe pulled up to the fast food joint Tuesday in Rialto, CA -- unclear what their whole order was, but eyewitnesses say it was a LOT of food.

We're told they did get some happy meals as part of their big order, so it's possible their kiddos True and new baby boy were in tow ... or at least waiting for the goodies at home.

KK and TT have been together quite a bit recently -- she was right by his side earlier this year when his mom died after suffering a heart attack. They both were spotted landing in Toronto, with Khloe being there to comfort him during his loss.

Tristan also showed face at Khloe's BFF Malika's birthday party, an event that most of the Kardashian sisters attended.

She also posted a super heart-warming birthday message about him last month, calling him "the best father, brother & uncle."

Looks like things are going well ... and we're lovin' it!