web page history

I think my sons are going to porn sites but they know how to delete history how can I retrieve it to check what they have been browsing

There are ways of doing this, you could also install some kind of parental monitoring software to monitor FUTURE activity.
but here is a way of retrieving old history information.

NOTE: software such as the one listed below may trigger your anti virus due to the type of software that they are, the one is listed below is a example and if you don't trust it please use google to find one that you do.
Index.dat reader search results

Browse the Old Index Files

Step 1
Click on the "Start" button in the lower left-hand corner of the computer screen. In the window that pops up, highlight and click on "Search" in the right-hand column.

Step 2
Select the option "All files and folders" from the search list. It will be the third choice from the top.

Step 3
Enter "index.dat" in the first text box on the new screen. The label above the text box states, "All or part of the file name:"

Step 4
Tap the double arrows to the right of the words "More advance options." Make sure there are check marks in the boxes for "Search system folders," "Search hidden files and folders" and "Search subfolders." Click the "Search" button. A list of files will appear in the window on the right side of the box.

Step 5
Minimize the results window by clicking on the box in the upper right-hand corner of the window that looks like an underscore.

Step 6
Open a program that can read index.dat files like Super WinSpy. It may be necessary to use a search engine to locate software that can perform this task. Once the program is installed, start it and read the index.dat files.

Step 7
Locate the desired history and restore by using the appropriate buttons in the reader program.