Since we don't have the option of riding buses in town we just take the kid that is actually enrolled in the elementary school system to school and she eats breakfast there so she can enter the building 15 minutes before school starts to go and eat. She often is hard to get to school early so she gets an Oatmeal bar and milk or juice in the van on the way and gets there right at 8.
The other child is in preschool and is not an actual enrolled child in the elementary school system so absences and tardy's don't really count against them. When the older child gets to school at 8 then the other child gets to pre-school about 8:15 and goes right in and eats breakfast with his friends.
If we had the option of a school bus we would let the pre-school child ride and not the older one. The little bus is only kids that are small and there is at least one teacher on at all times. They know the families and will not let the child go to anyone not known, they are only in town and not out driving around all over the county. The bus drivers are never left alone with the little kids and I feel that would be the safer situation.
With your situation I would ask around the K class to see if anyone in my area is driving and offer to give them gas money to do a kid pool (car pool for kids). You might even find another mom in the same situation as you and you can trade off so you can each stay home for at least a week at a time. Otherwise, for the safety of your other little ones I might say put both on their buses and hope for the best.
Next year hopefully both kids will be at the same school and you'll be able to take both at the same time.