If you have a good consignment store in your area, try with about 20 pieces. Make sure they don't have stains. See how the owner treats you. Consigners are only as good as they treat the people who bring them business. If they are bad to you, they will go out of business.
The nice thing about consignment is that you are helping moms who want nice things but want to buy gently used. You are also helping a small business STAY in business. You can either take money or take credit for buying clothes for your own growing child.
I always had trouble finding boy clothes because little boys are SO hard on their clothes, LOL! There were always lots of little girl clothes...!
Don't give the consignment store all 400 items at once. That's too much. See how it goes and dole them out. I doubt the store would want that many items at once anyway.
If you have a good relationship with consignment, you'll make a lot more money than if you have a yard sale. People just won't pay much for clothes in yard sales...