Holy lack of basic awareness Batman!
You cannot actually be serious?! You think £400 a month for twins should cover their portion of rent, bills and food?! That's what child maintenance is for..,clearly there wouldn't be enough there to even cover standard clothes...never mind school uniform! Also the whole "I pay what CMS says" is very telling.
we all know a child costs more than £200 per month to maintain, you should be offering to pay more or buy them extra clothes if you were any kind of parent.
I cannot imagine letting my child go without because CMS says it's ok.
A decent pair of shoes is around £40, warm jacket for winter £30, bags they'll actually like that will fit their needs £12 ( all that increases drastically as they get older though) school branded book bag £15, school branded jumper £15 each - you need at least 2...5 polos and 3-4 trousers/skirts you can get for around £30 - and she's got to do that twice!! That's going by the cheaper options as well and not even including socks and pants, which most of us buy new for the start of the school year.
My DS secondary school uniform was over £100 just for the PE kit! We've been lucky that his blazer has lasted a couple of years each time but his uniform is still over £100 a year for the standard shirts, trousers and shoes.,,refuses to wear a coat (🙄🤷🏻♀️) which we have to buy anyway...to be shoved in the bottom of his bag, left on the bus or otherwise abandoned! And don't get me started on lost ties!!
The long and short of it is you should be paying at least half for ANY school items and trips...because no, your measly £200 per child a month does not cover large expenses like school uniforms and school trips.