The Secret Summer by Ruth Chew, 1970.
This was one of my favorite books when I was a kid. Its original title was Baked Beans for Breakfast, but I think the paperback copies are mostly under this title.
Joe and Kathleen, who are brother and sister, are disappointed because their parents are taking a trip to Europe without them instead of taking the whole family to the lake like they usually do in the summer. Worse still, their babysitter favors their younger siblings and is very hard on them. The babysitter is even cruel and neglectful by making the older siblings sit outside in the heat all day while she played with the younger siblings inside, not even allowing Joe and Kathleen to come inside for water. Finally, Joe comes up with a plan for himself and Kathleen to escape their babysitter by going to the lake by themselves.
They leave a note for the babysitter saying that they are going to visit their grandmother and take a bus to the lake. Thus begins the adventure! The children camp out in the woods, trying to avoid people who know them and might tell their parents or babysitter where they are. They do make some new friends, including a nice older lady who hires them to help her with chores in her big, old house. However, as well as Joe and Kathleen take care of themselves, they can’t plan for everything. What will happen to the children when an emergency rescue means that their secret will be discovered?
Ruth Chew may be better known for her fantasy stories, but this was the first book of hers that I encountered. It would especially appeal to fans of The Boxcar Children, another story about resourceful children who are trying to survive without help from adults.