From what I remember, it was about a boy and a girl who lived with their relative (aunt? grandma?) who didn't treat them very well. Their school had a trip to the seaside and because she wouldn't let them go, they secretly saved the money to go themselves. When their grandma/aunt/whatever found out she was really annoyed, and I think she punished them severely, because it made them run away from home. They ended up running away to this huge hollow tree they had found in the woods and played in sometimes. They snuck out during the night with all their possessions and ended up living inside it whilst one of their richer friends helped them by bringing supplies, until one day she got lost in the woods in the rain whilst trying to find the tree and became really ill.
That's all I can remember, some of it may not be 100% accurate as it was ages ago. If anyone can help I'd be very grateful
