jas,
Yes, long time no "see". Bz with work and no time to "talk" too much.
Jadon's bus ride home is about 45-50min. It can be earlier sometimes cos some parents/grandparents will fetch the kids themselves. For pre-N, there is a bus attendant, besides the bus driver, who will make sure that the kids don't play a fool on the bus.
Jadon's class starts at 8am and there will be teachers around from 7.40am. From my observation, the bus will normally fetch the kids there by 7.50am and the driver and attendant (and principal who will be waiting at the gate every now and then) will bring the pre-N kids to the classroom. For the older kids, the bus driver just ensure that the kids goes into the school cos they'll know how to make their way to the classroom. As such, I think most schools will take extra care with the pre-N kids and will not leave them with no one to jaga. These schools have been running their business for the longest time, so they're really more orderly than we imagine them to be.
Actually, taking school bus can be quite fun. As I recalled in the past when I took schoolbus in Pri sch, we always fool around on the bus

We also hv bus party on the bus. There are friends to play and interact with. So nice! Just that the journey is longer.
What time does your MIL set off with Jadelle to school? Can check with the bus driver roughly how long will it be by school bus and if the timing is not very much longer, then one possibility is that she can go by schoolbus and then come home with FIL. That way, your MIL will not be around in the school's premises to interfere with things.

As for crying kids on the bus, well, I hv witness the pre-N kids alighting the bus on some mornings crying with puke on their clothes. But that was in January. The same kids have stopped crying since. You have to admire their adaptability. And they can do it with lots of encouragement and psychoing. Jadon's teachers commented in the past that everytime they begin to sing the goodbye song, he will pout and want to cry cos it's 1. time to go home and/or 2. time to be on the bus. The teacher will psycho him and tell him how good and brave, etc etc he is to take the schoolbus and after a while, he got a hang of it. The bus attendant will give him a sweet also. For my part, I spend several nights re-enacting a typical schoolday with him with his lego set and toys, from the point we leave home, drive to school, teacher tells story, going for music, playing slide at the playground, going for recess/snack, singing the goodbye song, boarding the bus with "friends", his friends alighting the bus complete with all the "goodbye and see you tomorrow", and eventually him alighting the bus (he's one of the last few) with his grandparents there waiting for him at the foot of the block. It's crazy, but it works. Mentally prepares him for what to expect. After abt 2 rounds, he can re-enact the typical schoolday by himself. That's when I can safely say he already knows what to expect.
If want to start schoolbus, must start fast. If the kids get an idea that the parents/grandparents will be fetching them, then it'll take a while longer for them to change that mentality and accept that they will be taking the schoolbus.
Whatever it is, all the best. I'm v sure Jadelle will cope fine with the right guidance from you.