Why don't you look at your current outgoings.
Net income.
Then subtract groceries, utilities, phones, wifi, annual car costs, other transport, council tax, sky tv, buildings and contents insurance, fuel, then subtract 2k a month for fees, then say a hundred extra for extras. Then factor in things like clothes, clubs, uniforms, pocket money etc.
Then also remember things like school trips, if they get one coming in each at up to several hundred, per each of them per year, can they go? Birthdays, Xmas etc, additional unexpected costs like boiler repair or car repair or new tyres.
Honestly I'd save the money for their university education.the government only gives loans for kids of up to 3.5 k to live on if parents earn over 30k jointly and this does not usually even cover rent if they live away from home, never mind books they need to buy , food and utilities etc. So you will be probably subsidising each to the tune of up to four grand each year. I'd save towards that, put them in state school and pay for tutors as is necessary.