I have been so lucky over the years that people have given me clothes for the kids.
If they are too big I put them up into the top of the cupboard, in the little girls room.
I arrange the clothes in size order and season, so all I have to do is go to the pile I need.
I usually start at the oldest girl and work my way down to the youngest or vice versa....just easier that way.
Taking the clothes to the next child and seeing if they need or want it. If not then I place back up until it is needed.
After I have done this, if there are any clothes that the youngest girl has outgrown and I can pass onto the boy, I do. But it is very rare.....
I do buy things for the kids if I don't have it in the wardrobe........If I have to start writing things down I will get an envelope and write down who it is for and size.