I've had to do this kind of planning before too haha! I was very fortunate that the bus driver offered to buckle and unbuckle my dh boy for me. So I just had to get him to the bus. If they stop in front of your house, you could carry the young one with you in the warmer months, and in the winter either utilize a playpen with a monitor, or a stroller like another poster suggested. When it's not too cold out you should plan your outside play time around the bus schedule.

I'm finally at the stage where I have all 4+ so I drive them all to school and the 4 year old comes with me everywhere. It's a different set of struggles haha... Younger kids need to be watched so closely, older kids have attitude

I hope your system works seamlessly for you when school starts!