You might find the following method useful in limiting your child's use of WhatsApp Web or by distracting him/her from it.
[Works only on Google Chrome]
STEP 1: Locate the LOCK near the URL Bar
STEP 2: Click on the LOCK button and the following window should drop down
STEP 3: Select the feature "NOTIFICATIONS" and the following should appear
STEP 4: Select the "Always Block on This Site" feature
Now, the messages will stop appearing on the side of the screen when a new one comes in. This is not a perfect solution to prevent your child from using WhatsApp Web as this only disables the notification from appearing and could only possibly keep him/her from being distracted.
As to a question posted about how do prevent legitimate sites from getting banned, I can only suggest the following method/s:
[Done on Mac OSX Yosemite]
STEP 1: Start and Administrator Account by following the instructions as given on this web page.
CLICK HERE FOR THE LINK
STEP 2: Visit "System Preferences" using an administrator account created
STEP 3: Locate the "Parental Controls" setting
STEP 4: Unlock the feature with your administrator name and password
Enter your password/username
STEP 5: Now, you can administer Parental Controls on your child's MacBook
I would recommend that Parents should fully authorise their child's usage of 3rd Party Software and not limit the software/source of the software in which they would download. This is as teachers may ask them to download software during lab sessions or in class and that they would then need your authorisation. The best solution I can offer is to ensure that you conduct "LD checks" whenever you wish, even though the school does on a random basis by clicking on the [Logs...] button at the bottom of the page to view the software, application and websites visited.
Besides, students have signed an Acceptable Use Policy (AUP) with the school pertaining the utilization of the learning device and the usage of other computing devices in his or her possession and to use the school's internet responsibly. You may refer to the AUP HERE
Ultimately, you would have to have absolute trust with your child about this matter. If you discover that your child still has problems managing gaming addiction or viewing other inappropriate material, you may contact his/her form teachers and discuss the best option/s available.
If you have any questions, feel free to reply in the comment section below.
Ng Wen Bin, Justin
School of Science and Technology, Singapore,
2016 Parents Engagement Session #4 Facilitator