Like you I would not do well with needing to rush back to meet the boys parents after the morning pick ups.
So, I think it is best that you give an honest thought to how much time you need to not be rush, to account for traffic, to account for having to chat with teachers and other parents at the school and leisurely get back home. Let the boys mom know now that you can only provide care for her from from X:XX onwards and for her to decide if that will work for them.
They will either realize it is just too many moving parts to pull it off smoothly at your place anymore or they will willingly accept that they need to pick up and provide care until that set time.
BUT....BIG BUT let them know that they CANNOT arrive at your place before that set time. It will just get stressful to arrive home each day to them sitting in their car in your driveway. And it will just create a grey zone of you being home some days early and other days just in time for that set arrival time and then they may start to plan for you to be there at the earlier time and it will just get messy. They NEED to plan to have somewhere else to be until that set arrival time. This needs to be made clear.