I'm currently watching the anime カードキャプターさくら because ケロ speaks 関西辯, which I want to improve, and since it's aimed at kids, I know it won't be too difficult. (And yet!)
At one point he says, "感心しとる場合かい", then "はよせんか！". I think the last sentence means, "Quickly, (won't you) do something!", but I'm not sure how to break down "せんか".
Is it: "早くしない(の)か"?
I'm having trouble finding an answer online because of the short length of the expression.
Is it: "早くしない(の)か?
Yes, except there's no の element here, it's just 早くしないか！ as in a demanding 'Hurry up, will you?!'
The negative ん ending comes from the old ぬ and する has the old negative せぬ, with ん it becomes せん. As for はよ, it comes from a chain of sound shifts going はやく -> はやう -> はよう -> はよ - even in standard language there's a remnant of this in おはようございます.
Remember that children usually have an intuitive grasp of the language far beyond a beginning learner. Pretty much any native Japanese speaker can understand this, much as pretty much any native English speaker can understand 'Y'all ain't seen nothin' yet'
