I have sold my kids clothing on Ebay, Craigslist, at Yard Sales and at Consignment stores. By far, the easiest way to sell all of it is by taking it to the consignment store.
You wash it, load it up and take it in. You get to shop while they look at your stuff. Easy. Right? Definitely. Do you get what you should money wise. NO WAY! Especially lately. If you opt for store credit there is hardly anything worth purchasing in the store. If you opt for the cash, look at what they actually bought from you. Most times they buy a ton of stuff and you take the money, not realizing that they pretty much just paid you garage sale prices. They used to give an itemized receipt for what they bought, but, of course, they no longer do that because they would be exposed.
I have been opting for Craigslist and I sell my stuff in a lot by size or season. You have to take pictures, answer tons of emails and arrange for pick up with the prospective buyer. A little bit more work, but you most likely can sell your stuff for $2-$3 a piece.
DON'T GO TO ONCE UPON A CHILD ON VALVISTA AND BASELINE!!! THE NEW OWNERS ARE BEARS! THEY PAY YOU NOTHING-THEN ALMOST EVERY CLOTHING ITEM IS OVER $5!!! I can go to a good sale at Target and Old Navy and pay less than that!!