When my son was going through a major growth spurt, I would just buy him one pair of jeans and one of khaki pants, do laundry each day, and he outgrew them before he wore out either pair.
For my oldest who has been the same size for several years, she needs less money. First, because she hasn't outgrown all her stuff, and second because she is much more gentle on clothes so they last longer.
If your kids have had a "foot growth spurt" over the summer, they may need a bit more money. Our local schools do not allow flip flops or shoes without a heel covering.
We don't measure equality based on total price because my son has large wide feet and needs expensive shoes to ensure he is comfortable and safe. I try to keep equality in the sense that each kid has about the same number of things.
In other words, 10 school outfits, two pairs of comfortable school shoes, gym shoes, and two "looking good with friends on the weekend" outfits.
I know the OP said budgets were a new thing, and I want to encourage you to talk about the financial changes with your kids and explain that everyone is making different decisions about money in the family. My teens are much more willing to "buckle down" when they understand the why of it and see it as something everyone is doing, including mom and dad.
When we patttern the behavior, without whining, it helps them get in the right attitude. I am a former high school teacher and I encourage you to sit down your kids and let them see the family budget and understand how much living really costs. Most teens really don't know how much things like health insurance, car insurance, federal tax, state tax, and water bills actually are, and they can, via lack of knowledge, feel like parents are being stingy when in fact they are being prudent.
Another idea that my mother used on us was to determine what the purchases for the year tallied at our local inexpensive "no name" shops. If we wanted a designer label, we were welcome to come up with the difference with money we had earned or with christmas/birthday money. Suddenly, when I was paying a 50% mark up it wasn't as important as when mom was footing the bill.
Lastly, my kids regularly get stuff at secondhand stores and garage sales. They understand that because we live frugally we can afford to do Disney and give money to charity. They know that there is only so much family money and that if we spend a lot on clothes, purses, cologne there isn't as much money left for the other stuff.
We have priorities as a family that the kids help design so they really try to keep costs low. We give to several charities that the kids helped choose for our family. It means something to them and so they are willing to sacrifice in other areas.
--Eeyore's Wife