A couple of Members have asked this question before. There is no simple solution and no app, but there is a workaround.
Before handing over the iPad to your kids you visit a page that is known to you and you note the time.
You hand the iPad to the kids, telling them that you'll be monitoring their use of the iPad and that they must not delete the Safari history or they will no longer be allowed to use the iPad.
Now, when they return the iPad to you, check to see if that page you visited is still there in the history file and at the time you visited it. If your kids have deleted the history file and then, after visiting sites you don't want them to see, revisited the page that you initially visited, it will have a different time associated with the visit.
Tim