They hit up the theme park Sunday for the third time in a month. Kourtney's 2 boys, Reign and Mason, tagged along with Travis' kids, Alabama and Landon.

They already seem like a blended family ... all smiles as they jumped on rides at Thunder Mountain and Splash Mountain.

Travis was clowning with his kids and they all mugged for pics during their day at the park.

It wasn't your average fam, for sure. Two bodyguards kept a watchful eye on the brood as they meandered around the park grounds.

They also had a VIP guide ... the cost is crazy -- $600 an hour with a 7-hour minimum.  One thing's for sure ... they can surely afford it.

Travis and Kourtney began dating in January, but they've known each other forever.  There are all sorts of rumors they'll get engaged. Who knows, but it wouldn't be surprising.