Using pound-cake variation on side of box (add instant pudding and 1 extra egg), prepare cake mixes according to package directions, and bake in four 8-inch square pans. Let cakes cool completely.
Spoon 1/3 cup frosting into a resealable plastic bag and set aside. Tint remaining frosting pale yellow. Stack two cakes, with yellow frosting in between and on top. Measure in 1 inch from edge of third layer, and cut wedges off layer to create lower slope of roof. Place trimmed third layer on top of stacked cakes. From the last layer, cut 1 inch off opposite sides of cake, leaving a 6-inch wide piece of cake. From center, cut a wedge off each side to bottom edge, creating the upper slope of the pointed roof. Stack last trimmed layer on top, using yellow frosting to secure.
Frost entire cake with yellow frosting. Starting at one corner, place fruit-leather strips vertically over sides of cake to create siding. Gently press into frosting, overlapping slightly. Starting at bottom edge of roof, place breakfast cereal in overlapping rows to create shingles. Snip small corner from plastic bag with white frosting. Pipe doors and window on barn.