phineas and ferb literally actively TELL linda that candace is telling the truth in many episodes. often they tell her "we built a rocket today" or some shit. it's linda who doesn't take any of her children seriously; she believes that candace makes things up or exaggerates and that phineas and ferb are playing pretend. on top of everythinf else going on i can NOT accept phineas and ferb slander on my dash today

Also 1) Phineas and Ferb literally have no control over the fact their inventions disappear and 2) They actively try to help Candace "bust" them every time there's an episode where she takes extra measures to do so because they want their mom to see what they made!!

Not only this, but Phineas often expresses remorse when Candace is disappointed in the inventions disappearing. They genuinely love their sister, and even though what she’s doing would likely get them in trouble, they support her in her endeavors. Not because they want to get in trouble, they never see anything wrong in what they’re doing. They just want Candace to be happy.