Found: Classic Novel

I'm trying to recall a book (fiction) about a well to do family who have fallen on hard times.
There are 2 or 3 youngsters (2 girls and 1 boy).
Mother is dead..Father is a writer. The kids lock him in a tower to help him get over his writers block

I Capture the Castle by Dodie Smith

Yes that's it! Thank you Marissa!

Finally, a book I know, but, alas, someone else knew it first. It's a really good book. She also wrote The One Hundred and One Dalmatians. A much better book than movie.