SOLVED. Children's. magic of some kind, and a turtle (or tortoise) is involved. I think it can grant wishes. There are four or five children. [s]

[Imaan]

Okay.
It's a fiction book, and quite easy to read - no overly complicated language.
There is magic of some kind, and a turtle (or tortoise) is involved. I think it can grant wishes.
There are four or five children. I'm almost certain that one of them is called Martha.
The children are on holiday, or a weekend away, or something. I'm pretty sure of this too.
At some point, (and this is a weird one), they are all hot and tired from walking, and they reach shade at last, and one of them says, "coolth," and insists that it should be a word, because "warmth" is a word, so why not?
The whole thing is written in a bit of an Enid Blyton kind of way? I think.
It's not "The Five Children and It" by E. Nesbit.
Thank you.

[Anna]

Sounds like Magic by the Lake by Edward Eager.

[Imaan]

YESSS! Thank you so much, I've been looking for this for ages! Thank you thank you thank you XD !!!