Swallows and Amazons is an amazing book and, in regards to your first request, can be avidly used to display didacticism. Let's begin with the definition of didcacticism so as to be completely clear:
Didacticism: communication that is suitable for or intended to be instructive
Quite simply, a work of literature that displays didacticism is one that is meant to teach something. That work of literature, then, can be called "didactic" in nature. In reality it is the later term (didactic) that is more commonly used than the former term.
There are two ways that Swallows and Amazons can be didactic. (Because I am not sure which of the two you mean, I will give a short idea behind both.) First, Swallows and Amazons can teach the reader through the plot of the story. The Walkers (the Swallows) and the Blacketts (the Amazons) can teach the readers to enjoy a friendly rivalry, but never to take it too far.
But the big hills up at the lake helped to make him feel that the houseboat man did not matter. The hills had been there before Captain Flint. They would be there for ever. That, somehow was comforting.
The story can teach respect for others' property (as seen in the true danger of the burglars and the disruption of the fireworks shot off on the top of the houseboat). On a lighter note, the story can teach the importance of adventure in a young person's life.
Swallows and Amazons for-ever!
There is no better quotation than this in order to reference that excitement of adventure. Remember, this story was written by Ransome because of a fun summer teaching children how to sail.
Second, Swallows and Amazons is didactic in that it can be used to teach students about many elements in works of literature. It is a perfect example of conflict (Swallows vs. Amazons), symbol, character development, plot development, and especially allusion. Let me elaborate on the last entry here. Swallows and Amazons employs continual allusions to the novels Robinson Caruso and Treasure Island. (An allusion, of course, is an indirect reference to a literary work.) The campers always call their lemonade "grog," admit to practicing "piracy," turn their Uncle James Turner into the notorious "Captain Flint," sail boats, find "treasure" of Turner's memoirs, force characters to "walk the plank," etc. The allusions alone would be a wonderful topic for a student essay! Thus, this shows that Swallows and Amazons is a perfect example of didacticism.
In regards to your request for "child agency" in the story, I can't quite figure out what you mean by this; therefore, I will take a couple of different approaches in hopes that I hit the nail on the head in regards to what you are looking for. There aren't any agencies for children focused on in the story; thus, I can think of two things you might mean. If you mean "childishness" in regards to character, then one needs to look no further than the rivalry in the title. Further, the Blacketts setting off dangerous fireworks on the roof of James Turner's houseboat is a very specific example of childish antics. If you mean an "agency" or a "group" formed by children, then we can look at the group called the Swallows (which are the Walkers) and the group called the Amazons (which are the Blacketts). Together they exemplify the "piracy" of the summer with their adventures and pandemonium.