7 kids + 1 bus driver = 8 people x 2 legs each = 16 legs
7 kids x 7 backpacks = 49 back packs.
Each backpack has 7 cats. 49 backpacks x 7 cats each = 343 cats x 4 legs each = 1372 legs.
343 cats x 7 kittens = 2401 kittens x 4 legs each = 9604 legs.
Total legs: 9604 + 1372 + 16 = 10,992 legs.