"a quiet head"? I need a more natural sounding equivalent

So that the employees may work through the summer with peace of mind.

This assumes, of course, that the bank isn't holding the children hostage in the camp pending the successful completion of the project.

The expression take your mind off (somebody/something) may fit in the context:

• to cause you to stop thinking about who or what is causing worry.

  • That's the good thing about helping other people – it takes your mind off your own problems.

(Cambridge Dictionary of American Idioms)

• The fact that the bank will take care of the children will allow employees to take their minds off, be more relaxed and focuc on their activities.

The bank put their fears to rest.

I can't think of a common phrase that expresses this sentiment exactly, so I think you can construct something like "free to focus (on work)". Here's an example specifically regarding a student not worrying about financial aid.

Other suggestions, such as "peace of mind" or "putting fears to rest" mean more an absence of emotional turmoil. If you want to indicate that employees are free from distractions rather than distressing emotional states, don't use those phrases.