Probably 立てる #4. to make; to establish; to set up; to develop; to formulate
Then, the sentence is 予定立てているんのか えらいな 
Your translation is correct. 
“It is great/admirable that you have built/erected a plan”
Although I think these are two sentences.
“Oh, you’re making a plan? That’s great.”
The よていかー is more like, “A plan, huh?”
Other than that, it seems that Yotsuba has heard the word before, but didn’t really know what it meant, and now she related the action of planning to the word, so she’s like, “So that’s what it is.”
You can more or less figure this out since she uses the word a lot from there on.
I think it may be implied that either Dad had already told her what they were doing today, and he’s just reminding her while using the new word. Like, “The plan for today is to go shopping.”, or that he had already planned it, and is just telling her now.
And she’s like, “So that was the plan?” or, “Is that right?” (Basically the same.)
“じゃあかいものにいかないと” “Then, we have to go shopping.”
Remember the part where Yotsuba has to cross the bridge to give 風香 some milk?
おかしは しまっとこー → おかしはしまっておこう (Volitional; It’s always between 9 or 10 
)
“Let’s put away the sweets.”
Remember っと can be ってお, you also usually see it a lot with っておく.