Back to School Means Back to Shoes
Children's feet are extremely sensitive. While some don't like going without shoes because of sensitivity when they are barefoot, most enjoy the freedom of not being enclosed in a shoe. When he spends the summer with unconstrained feet, they adapt to the loose fit and freedom. Most schools require that students wear closed shoes. The kids quickly go from freedom to foot captivity. They'll complain that their shoes are hurting them, but the likely truth is they feel suffocated.
In the weeks before school start it is important to prepare your children to return to shoes. Try on their current school and athletic shoes to ensure they still properly fit. Kid's feet grow fast and you need to make sure that your kids aren't justified in their complaining. Be ready to take them shoe shopping!
As school approaches, gently remind them that they will need to wear appropriate shoes for school. Have them break in new shoes prior to school to make sure the school year starts without a hitch. By the time school starts, your kids and their shoes will be ready to go.
I'm not naive to realize that there is some wishful thinking in this plan...after all I have three kids myself. Try not to just throw your kids from zero to school shoes and the transition will at least be a bit better. Just remind them...they still have weekends!