Why can the koyukkuri eat them and still be fine? Before translation, I thought it was because they were different flavors. (The things on the packaging look different.)
There's a type of candy product in Japan which is basically three candies - one of which is sour (there's no way of telling which one without tasting them). I assume that's what we have here.
Panel 1: Mother Reimu: "There are exactly three!! To make sure, mother's going to 'taste for poison'!" Marisa: "Nice smell~..!!" (The 'ne' for emphasis is slurred.) KoReimu: Yuuu~~
Panel 2: Mother Reimu: Crack! Muunch, muunch... Marisa: "So? Tashty!?" (Tasty is, as you guessed it, slurred) KoReimu: "Hurry, I wanna eat!!"
Third panel: Caption: Jackpot
Fourth panel: Mother Reimu: *Goho Buhoo* (Buhoo = boo hoo, as in crying, so... yeah.) "N-no... good, little ones! This candy can't take it easy at all!!" Marisa: "Yuuu, it's alriight!" (Alriight is slurred) KoReimu: "Yuuu... But it's making/has a good smell?" Caption: Sour/acidic drool