My ex pays half of all bills related specifically to ds. We both work full time, have mortgages, pensions etc.
So he pays half of his clothes, shoes, tech, books, big purchases like bikes, a quarter of my electricity bill & my water bill. He also pays half of school related stuff like GCSE study trips, equipment, sports lessons etc. My ds is 14.
At the moment it is averaging £190 a month, so roughly £45 a week. I also have £84 a month child benefit which covers ds' food. Giving me £275 a month for one child. For that, ds has a nice life, all the clothes and activities he needs. Two sports lessons at the weekends.
£175 per child per month is tight, especially as you will only get £57 CB for the second and third children. So that gives you £724 per month. But you can do hand-me-downs between children which cut some costs. You should be able to cope fine.
I cannot understand how you spend £1200 a week. That's a crazy amount of money. Do you drive a Bentley?