Cell C has that Giga Pack deal:
• 50GB Any time data
• 150GB 12-6a.m.
• Prepaid: 365 days to use it up
• R 1 000 once-off
This translates to, per month:
• 4 GB anytime data
• 12 GB 12-6a.m. data
Ask around if you can set monthly limits using the Cell C app.
This should save you an arm and a leg, just confiscate the phones at night so they do not stay up surfing the web.
Hope this helps.
Edit:
Assuming two kids:
The vodacom deal costs about R 1 000 after 5 months, making Cell C's cheaper:
Vodacom Deal after 2 years:
• R 5 000 total
• 120 GB anytime data
• 120 GB night data
Cell C deal after 2 years:
• R 4 000 total — each kid has his/her own data
• 100 GB anytime data
• 300 GB night data
A big consideration would be the upfront payment.
Second edit:
I just checked on my app and you cannot set a monthly limit. Instead, there is an option to transfer airtime, perhaps someone may chip in and let us know if it is possible to transfere data; this way, you could better manage both your kids Internet usage.