Toss the oats with the butter on a rimmed baking sheet and spread in an even layer. Bake until toasted, 18 to 20 minutes. Let cool completely, then transfer to a large bowl. Stir in the almonds and cashews.
For the bars: Line a 9-by-13-inch baking pan with foil, leaving an overhang. Coat the foil with cooking spray.
Combine the sugar, butter, honey, salt and vanilla in a medium saucepan. Bring to a boil over medium heat, stirring occasionally; the mixture will bubble and foam. Boil for 30 seconds, then pour over the oat mixture and stir. Let cool slightly, then stir in the 1/2 cup sprinkles and crushed animal crackers. Pour the mixture into the prepared pan and press firmly with a rubber spatula to spread in an even layer.
Let the bars stand at room temperature until completely cool and firm enough to slice, 1 to 3 hours. Lift out of the pan using the foil overhang and cut into 20 bars.
Line a baking sheet with waxed paper and set aside. Put the frosting in a microwave-safe bowl and microwave in 5- to 10-second intervals, stirring between each, until fluid but not too loose. Stir in the coconut oil. Dip the bottoms of the granola bars in the frosting, let the excess drip off and put on the prepared baking sheet to dry until the frosting is set, 3 hours or up to overnight.
Store in an airtight container at room temperature for up to 5 days.
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