Definitely old-fashioned term for icecreams.
I've been rereading EB with my eight year old first the first time since I was his age, and what always strikes me about the Famous Five's food/drink arrangements now is that they are continually setting off on long hikes/cycling holidays in hot weather without anything to drink with them, and relying either on helpful springs or little shops for drinks (and ices).
Of course bringing glass bottles of lemonade or their favourite ginger beer with them in backpacks would have been incredibly heavy, and there simply wasn't the culture of bottled water, either way, in the 1940s, for obvious reasons. I think it just struck me because the books take such delight in nice food and the Famous Five get through so much of it! that the reliance on very occasional lemonade/ginger beer supplies struck me, especially when they're cycling or hiking for hours every day.
(Also that there's an assumption that random farmhouses would be able to sell them food...?)
But the real delight is having to explain to my eight year old why, when the Five Find Outers befriend Ern, the village policeman's working-class nephew, and he comes back to Pip or Fatty's house to tea, he eats in the kitchen with Cook and the parlourmaid, while the other children eat upstairs in their playroom. 
My eight year old has also pointed out that EB considers tomatoes a great treat, and it's true -- she fetishises them!