Book about children who get trapped inside a painting

I read part of this book almost 50 years ago and want to know how it ends! This is all I remember (or think I remember):
Set in England (?), maybe mid-20th century, possibly during WW2. In the story, a child (or possibly children) staying with a relative (?) notices that details in a painting on the wall of the bedroom he's sleeping in change from one day to the next.
I recall that in the painting there are details such as a river, a dinghy and also children, and that these things change over time - e.g. the boat moves and ends up moored further up the river. The child in the story somehow ends up in the world of the painting as well. And apart from the fact that all the children in the painting are under some sort of dire threat, that's all I remember.
Here's hoping someone knows this book and I didn't just dream up the whole thing!

Not Voyage of the Dawn Treader?

>2 BrainFireBob: Thanks for your reply. No, it's not that one, but thanks anyway!

Not Dahl, The witches?

Lively, A stitch in time?

68 Rooms by Marianne Malone? Not exactly the same details though.

That sounds eerily familiar. For some reason The Akhenaten Adventure is associated with my memory of the kids noticing the boat in the painting moved down the river then getting trapped inside. Maybe I read them both around the same time & my memory's overlapping, or maybe the Children of the Lamp series referenced your book.
Bookel mentioning (ah, now I see why the author is mentioned) Lively, A Stitch in Time also evokes memories associated with your book.
I think I also remember that at 1st they could go in & out of the picture but start to suspect that other children stopped being able to leave. I think there was also someone or something talking to them about the magic of the painting.