When you have 3, 4 or more children all the same sex and close in age it can be difficult to keep underwear and socks sorted and in the correct drawers. We all know about "Hand-me-Downs". Dealing with them can be helpful if you use a permanent marker and put 1 dot it in the waistband or tag of the underwear for the oldest child.
If your underwear doesn't have sizing tags, it's a very good idea to label them yourself. It makes it easier for you to know where to place it, in which child's drawer it belongs.
Overhead, above our washer and dryer, I had my husband install a shelf that runs the entire length of the washer and dryer. On this shelf I placed five mini laundry baskets the ones that are usually 10" x 12".
I organize kids socks and underwear so they don't get bunched up together. I use the bottom half of plastic coke bottles or liter bottles of other soda to do this. You can see through them, and they keep the drawer neat.
When my kids were little, I had four within 5 years, the last three were girls. As the girls started wearing panties, I put their initial in the back right under the elastic.