I would shop around via lots of insurers to see if you can get a better quote, a telematics box in your car would help, but they may struggle to identify who is driving. You have not said whether they have passed their test or not, so I am guessing not. Try www.quotegoat.com/compare-car-insurance-quotes/che.../

They have just popped up on my radar. They have sourced some good underwriters that appear to be giving good quotes for youngsters. Swap the whole policy and using your no claim may bring it the premium down substantially.
The other question is, why do you want them on your insurance? Again from that I am guessing it may well be because you want to teach them. You may find what you pay for insurance for them both may be better spent on lessons.