BTW
 

This technique isn't unique to girls; it is the standard baseball bent leg slide. See any baseball coaching manual: head back, hands up, initial contact points are calf, thigh, and buttocks.

Now I don't see any HS boys or adult men SP players wearing sliding pads. But I do see a lot of ugly raspberries on those exposed calf's and thighs. (OH,that testosterone driven male ego!) However, at the local sporting goods stores I see both mens and womens sliding shorts on sale.

Re: Sliding technique?
 

Well, now that you mention it, I've seen players, even at the major league (dare I say it?) baseball level have interference called for throwing their arms up while sliding into a base.

Point of the slide is that you do not slide into it leading with your feet, but as you say, "sit" down into or drop the tail end first.

I am very familiar with sliding pants, and no, I'm not confusing them with compression shorts as many do. You will note that the padding on sliding pants (along with the most-worn portion of the uniform pant leg) is positioned on the outer portion of the upper thigh.

BTW, don't you think for a second that you you were the only person here to coach. The only difference is the teams I worked with were boys and men. Used wrestling mats in a gymnasium while wearing shorts. It only took a session to get the point across of what part of the body you want to use for initial contact.