I would talk to the school and see if they can make an exception and allow you to park in the staff lot for drop off and pick up in light of your situation (easy for your child to see and walk to you) If it is only until he is 5 or 6...they may be happy to help.
I wouldn't recommend leaving the children in the van alone. Someone would see, and report, and even though it is very safe, you would look bad which would be particularly crappy as you are a care provider.