If you watched Phineas & Ferb from start to finish, you might have noticed that sometimes they outright tell their dad about their inventions or sometimes he can see it with his own eyes.
It's save to assume that if their mom saw what the boys were doing, she would punish them. Because why would Candace be trying to bust her brothers by telling her mom instead of dad? Candace knows that dad wouldn't react.
So, the question is, why does Phineas and Ferb's dad never react as you would think he should when he learns what his boys were doing? Why does he just go with it?