The Green Portal conundrum

Olive and Kuba never imagined that an ordinary holiday day could turn into a real adventure. Their grandmother lived on the edge of the forest, in an old, mossy house that seemed to hide a thousand secrets. They liked playing most in the garden, full of giant ferns and vines so thick they almost blocked out the light. One cloudy afternoon, Cuba noticed that one corner of the garden had never been thoroughly explored by them. He pulled Olive by the sleeve. - Come on, there's something there! - he whispered, pointing to the bushes. They slipped between the leaves and suddenly found themselves standing in front of an archway woven from living branches. A greenish mist shimmered gently through the centre of the arch, and a strange chirping sound came from within. - 'I think it's just another gazebo...' Olive began, but at the same moment the branches swung wider, revealing something neither of them had expected. Beyond the portal stretched a completely different world. Instead of a garden, they saw blue grasses, glowing stones and trees with silver leaves. Every now and then, strange little creatures resembling a mixture of squirrels and birds flitted between the branches. - Shall we go in? - Cuba looked at his sister with wide eyes. Olive swallowed her saliva and took a step forward. At the same moment, the portal flicked even more intensely and the shadow of someone emerged from behind a tree on the other side.... or something that seemed to be waiting for them. Would Olive and Kuba dare to cross the green portal? Or will they discover an even bigger secret hidden in their grandmother's garden?