You should be able to make use of the Lock feature so that this will be active even when signed out of the account. Using this feature, you can theoretically lock the setting so that other users of the iPad don't change it.
Visit the Search Settings page at:http://www.google.com/preferences
Turn on SafeSearch by checking the "Filter explicit results" checkbox.
Click Lock SafeSearch the right of the "Filter explicit results" checkbox.
Note: This is not foolproof.
On an iPad you may have better luck using the Website Restrictions feature built into iOS 7:
Look for the section entitled "Allowed Content:"
This is a free feature in iOS 7 and it is at the system level so it can block porn and other adult themed websites throughout the device without requiring the user to use a specific browser. So, it will block access even in 3rd party apps that make use of the Safari web browser. You can can manually block and allow certain websites, as well.
If you need more assistance with this feature or are unsure if you are using iOS 7, then please let me know.If you still have concerns, please do not negatively rate my answer, simply reply back before rating. I'll be more than happy to work with you further
. Hopefully, this info will answer your question. When you are satisfied, please give my answers a positive rating. You are judging my personal service only (not this website or your issue) and high ratings assure I remain in good standing.
Thanks and good luck,