I have three kids, as you may know, and I have made a tradition of sewing them a first day of school backpack.
I use the same pattern each time, Butterick 5054.
It's a little smaller than your average backpack and just a little bigger than a teensy person needs, but they can grow into it.
The Elder's is here.
The first day dress was a pattern that I made for The Elder for Kindergarten: The Birthday Dress from Oliver + S.
I may have gone a little overboard, and coordinated her first day dress with her backpack.
In my defense, she loves purple...