Our primary school age children do cost a fair bit. Per child:
£250/month wraparound childcare - 3x week each
Activities - £100/month for my older one who really likes being busy, this includes swimming, martial arts, two school clubs, musical instrument and Beavers. It's a lot less for the younger one who doesn't like organised activities as much
Food - really hard to estimate, I agree with PP that we make more of a conscious effort to eat well and a variety, we eat a lot more things like salmon and soft fruit, also one child has a dairy allergy so we spend on substitutes. I would say it's probably at least £100 more on food per month
Clothes - we get a lot secondhand but even so maybe £20/month, including school uniform. Less for the younger one as he obviously gets hand me downstairs
Then there's toys, books, misc technology (yoto player, tablet etc), birthday presents for friends, holidays etc
I don't think £600/month is far off actually.
Of course a lot is discretionary - we pay a lot for activities relative to many families.
And also a lot is in how you count these things - I see at least one person has said they don't have childcare costs because they are a SAHM, I personally would count the loss of that salary as the cost of the childcare but everyone calculates these things differently