I'm trying to identify the a children's fantasy book. My sense is that it's pretty old (before 1950) but I could be wrong.
At the beginning of the book, one of the boys in the group buys a little model ship. The first inkling he has that something funny is happening is that it changes size so it will fit in his pocket. Then in the rest of the book, I think it becomes life size and he and his brothers and sisters use it to travel places by magic.
One thing I specifically remember is that they they encounter someone who wants to take the ship away from them, but finds he can't. It turns out you can only acquire the ship by paying slightly more than all you have in the world. In the boy's case this was easy: he had gotten an advance on his allowance from the next week.