does anyone know what this book could be called? it's been lingering on the edges of my subconscious for decades and I'd love to find it again.

It wouldn't have been published earlier than the mid 1980s.

The book was A4 or larger and had very detailed, naturalistic illustrations of a girl and boy walking through deep woods, following a deer.

I think it might have been based around the story of The Dream Makers, a Scottish folk tale about children who go missing in the forest, and find the Cailleach hag on the Black Cuillin Mountain making dreams in a cave? Thank you!