"
built a roller-coaster,
And a beach in the backyard,
Drove cattle through the city,
And messed up the boulevard,
They took me back in time when we went to that museum,
They built 50 feet tree house robots,
But still you didn't see them
This part is simply stating some of the things that Phineas and Ferb have done, and then saying how Cadence's mom never sees them.