If you are going to be wearing a mask and the kids are not- I highly recommend you get N95 masks, as these are the only ones that will filter and protect you from the virus. I would even go as far as having a hand sanitizing station at the front of the bus so the kids can sanitize their hands before sitting down.
I would also recommend taking the issue of the cost of the masks and sanitizer to your school division, and if nothing comes from that, talk to the school trustees. There should be no reason that you should be out of pocket for your safety.
Anyways, I hope all works out for you and that you remain safe.