YAB a bit U
To put in a word for Ebay. I do sell children's clothing on Ebay, because I feel happier that I know the item is having a second life.
I don't make much money, but I regard it as a way of constructively decluttering and getting a proportion of the cost back for things. One third of new cost is my rule of thumb for lightly used mid-market brands (Boden, Gap, John Lewis and M&S) and cheaper items can be sold as bundles.
The way I see it, selling on Ebay (to someone who definitely wants that item) introduces an additional user into the life-cycle of the garment:
Purchased from new
Sold on Ebay to second user
Donated to third user
Recycled for rags
It isn't a huge amount of hassle either. Every so often I will spend a couple of hours photographing and listing up a big batch of items, which then just stay listed until they are sold. I then package it up and pop it in the outside box of the post-office as I walk past, which tends to be a couple of times a week. I also prefer the relative anonymity of Ebay.
If you donate to a charity shop there is a reasonable chance that the item will never get used again. People say 'give to charity' as if they are certain that poorer people are going to be getting/benefitting from those items, but the truth is that charity shops throw out a huge amount of goods or often send perfectly good clothes to the 'rag man'. This might include out-of-season items, such as a winter coat in the middle of summer. Obviously giving to a refuge or a baby bank is likely to be genuinely helpful to someone in need, but that is a bit more effort for the donor as baby banks aren't always easy to get to or are only accepting certain categories of donation at any point in time.
That's before you get to the charity shops which aren't really that charitable at all...Particularly in urban areas you can find shops that look and feel like charity shops but there isn't a registered charity number up anywhere and it is actually quite hard to pin down exactly what the funds are doing - there might be a vague mission statement up on a poster, but who knows how the funds are being spent? Don't get me wrong, I think charity shops are a fantastic institution, but there needs to be safeguards like registered charity status and they aren't necessarily the solution to our fast-fashion society.
Doorstep collection bags aren't always what they seem either - yes, some are for reputable charities, others are commercial organisations that then make money by selling the clothing in developing countries, sometimes for quite eye-watering prices 
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