I read a children's book when I was in elementary school back in the
1970s.  It was written either in the 1960s or 1970s.  It had to do
with a brother and sister that were sent to summer camp.  They didn't
want to go and once they got there it was raining and a mess.  They
met this witch and she turned their tent into a place with all sorts of
pinball games and everything a kid could want.  They then have a great
summer. Unfortunately, I do not remember the title of the book. Thanks
SO much for your help!!!

Hello.

I believe that you're thinking of "The Witch's Vacation" (1973) by Norman Bridwell.

The book was first published in 1973 and there were  multiple editions
in the 1970s and 1980s.


" The boy and girl love the witch next door so much that they hate to
leave for camp. So the witch goes to camp with them! "
source:
Alibris.com
http://www.alibris.com/search/search.cfm?qwork=7255970

"A little girl and her brother go to camp and the witch uses her magic
to help them to have the best vacation ever."
source: abebooks.com
http://dogbert.abebooks.com/servlet/SearchResults?an=bridwell&imageField.y=0&imageField.x=0&ph=2&cm_re=HP*Search+Box*Form&kn=camp&tn=witch's+vacation&sts=t

Used copies starting at $0.70 on Amazon.com:
http://www.amazon.com/gp/product/offer-listing/0590405586/

--------
search strategy:
alibris.com search witch camp

I hope this is the book that you had in mind.  If not, please let me
know via the "request clarification" feature. Thanks.

Thanks so much!  This is exactly the book I was looking for!!