It's not your first neg. Outrage usually accompanies the first.

Why are you only concerned now, more than 3 months later? Bit late to do anything now.

Were the socks listed in the kid's category or the adults category?

Buyers are notorious for not reading descriptions, so going forward it's always better to spell some things out in the headings.

I would have first contacted them and explained that you are sorry they are disappointed, but that the information was in the description and the socks were listed in the correct category. I would have asked them to please submit a revised feedback as you were not at fault. If the customer refused, then you can remind them that you have the right of reply.

As to the reply itself, I would have replied something like "Socks were listed in the Kids category and it was mentioned in the description. Buyer did not contact me with any questions prior to purchase". This puts the responsibility back on the buyer and most people can deduce that the buyer did not read the description so the review could be considered unfair.

The positive thing is that reviews are on a 12 month roll, so it will eventually drop off the radar.

I have sympathy, it does seem a bit harsh, perhaps a neutral would have been a little easier to take. But, I do agree with others, responding to feedback is never a good idea unless it shows humility and some responsibility. You don't have to mean it, how many restaurant reviews do you see where the owner has a generic response to criticism and t makes them look like they care. 

Selling on eBay can be unfair as the rights seem to be heavily skewed towards buyers and at times you have to respond in ways that you would rather not and have to bite your tongue so much it bleeds. But I guess that is business. If you had held off responding to the feedback, you might have had a shot at a revision or eBay removing it. But take it as a lesson, on eBay, you literally have to include every detail you can to avoid issues further down the line. You need to 'child-proof' your listings. Remember a lot buy from the App which does not always show the description easily. 

you could have gone back to them and asked them to revise it explaining why nicely.

but based on your rude reply if i was asked I would not change it.  Your replies to Feedback are critical, sometimes it is better to say nothing, then to get all uppity.

As you have replied to the FB, there is nothing you can do.  For your current listing....put the  actual size in the heading   (size 1-3)  not just  kids socks.

Try the link below to get eBay to remove the feedback.

https://www.ebay.com.au/sellerhelp/feedback