Tom Brady was one cool pops to his kids on Thursday ... 'cause he made a guest appearance on internet superstar IShowSpeed's stream!!

The collab went down as Speed was kicking off his "Speed Does America" extravaganza, where he is set to explore the country over the next 35 days -- streaming the whole thing for his millions of subscribers in the process.

Speed -- who was channeling his inner George Washington crossing the Delaware River -- pulled up on a boat to the dock outside Brady's Miami crib ... where the GOAT welcomed him in for a tour.

After getting over how nice Brady's pad was, Speed made his way into TB12's gym ... and he was challenged to bench 225 pounds -- all without warming up.

The streamer was able to get in one rep -- but Brady had to step in and help him on the second.

After doing some more exploring ... it wrapped up with Brady handing Speed a gift -- a signed 2002 Cristiano Ronaldo rookie card.

In case you didn't know ... Speed is the BIGGEST Ronaldo superfan.

"You come to my house, I show you around, I give you snacks, and I know he's your guy," Brady said of the gesture.

Like Speed said ... this is only the first day -- and we take it Brady's just the first of many collabs to come.