I'm guessing since the first book was in the 1940s, copyright might be expiring soon, and parodies do have some protection.
That sort of depends. As long as the owner of a work renews copyright, it won't expire. So if the owner was Enid Blyton exclusively, she might have passed on rights to her kids (she had at least had two daughters; not sure whether there were any more). If her rights to the books were never passed on to posterity, though, the copyright might expire, depending on law. In the U.S., it's the lifetime of the author plus 70 years. I have no idea what it is in the U.K.
If her rights were passed on to the publisher, however, that's another matter. At the time, the publisher was Hodder & Stoughton, but they last published anything from the series back in the 1960s. So they would have had to renew the copyright since that time even though they didn't publish anything from it again. What's more, Hodder & Stoughton are now owned by Hachette, so the terms of the acquisition would have had to specify that rights to
The Famous Five be passed to them . . . and then Hachette would have to keep renewing copyright for the series.
Ms. Blyton passed on in 1968; so if rights were hers still and
not passed on to her kids, it could be still protected for a bit. (If she were American, it wouldn't expire until 2038.) If rights
were passed on to her kids, it would depend on whether they kept renewing copyright through the years. If rights were passed to Hodder & Stoughton instead, there is probably a much greater chance that copyright could have been lost over the decades — I would venture it's unlikely a publisher will keep renewing copyright for something they really don't see any profits from anymore.
However, as you say, parodies are generally fairly well protected, so that could definitely be a thing.
