fantasy:brother and sis try to help creatures in another world

I read this book twice back in the early 80's (3rd/4th grade). All I can remember is that the brother and sister were sitting on a park bench in the rain when they were approached by a man who had an umbrella (or maybe they had the umbrellas). Anyway, they ended up somehow in this very colorful world and met a strange creature who they befriended. The strange world was in danger because of an evil being. It was really good...if this sounds even remotely familiar, please let me know!! Thanks in advance!

The Last of the Really Great Whangdoodles by Julie Andrews Edwards, maybe?

OMG!!!!!!!!!! That's It!!!!! I am so in love with you right now!!! LOL Did you read that book? I loved it and now thanks to you, I get to read it again! Thank you, thank you!! :)

Wow, well I'm surprised I guessed right. Yes I read the book last year, so had a hunch without searching at all that it was the book. Enjoy rereading!