Yotsuba is smart. When you want to solve a problem, you meet it head on! Daddy keeps thinking that if he asks Fuuka out in a sideways way, she's not going to turn him down.
Yotsuba knows (because I-- she!-- already asked Fuuka) that Fuuka's going to accept anyways. So she doesn't understand why Daddy keeps staring in front of a mirror and mouthing some words all the time. Daddy's not even playing with Yotsuba!
All that Daddy needs to do is ask, and Fuuka'll say yes, and everyone will get their happy ending! But since adults are complicated, (and Fuuka is turning into an adult, so she doesn't understand things the simple way anymore) Daddy keeps putting off his decla-- declara-- decla-something of love, Fuuka is getting sadder and sadder (she thinks Daddy doesn't love her) and I don't get to play at their house anymore (because Daddy thinks I'm going to tell Fuuka before he gets to).
And since Daddy won't play with me anymore, nobody plays with me.
You know what, I'm going to run away to their house, and tell Fuuka all about this! That's the right way to do it.
I'm sure they'll thank me later.