Op do you spend £500 or less per child for your children?
Remember to include:
Larger mortgage/rent payments due to needing enough bedrooms
More council tax
More utilities payments for a larger property
Childcare (do you work? Does he?)
Furniture
Soft furnishings
Household items like lights and lightbulbs for their rooms
Books and toys
Tech equipment
And that's before the more obvious regular expenses like:
Clothes and shoes including uniform and PE kit
School equipment including bags
Transport
Haircuts
Medical/health expenses (1st aid items, otc meds, sun cream etc)
Increased grocery expenses (not just food but also cleaning items, toiletries etc)
Items for clubs as well as fees
Birthday and Christmas items plus cost of parties - not just theirs but the ones they attend too.
Entertainment costs (streaming services, in normal times trips to swimming and cinema days out in school holidays etc)
You say you have 2 children of your own yet seem oblivious that £250 a month barely touches the sides!
The cms amount is a MINIMUM legal requirement. If he were still with his child's mum he'd be contributing a damn sight more than £250!
They move in with a man with kids and then start complaining that he has to pay for those kids, even though it’s the minimum. How very dare the mother of his child want money to bring up her child
Yep!
I was lucky insofar as it wasn't ex's 2nd wife was the motivator of his crappy attitude to maintenance it was all him! But I am very aware of many other cases where this is very common.
Ahh your ex is stiffing YOU - jealousy then!
if I didn't have kids my gas/electric would be the same I'd have the heating on for myself?
I see this and similar comments made on mn frequently. My dd moved out just over a year ago (she's now 20) and I am on a smart meter. She worked full time and is quite a sociable sort around here with friends she's known a long time and was rarely home. My energy bills have still dropped by almost half! Utter nonsense that they're hardly using any! My grocery bill by about 1/3 even though she seemed to rarely eat here. I think because it's a gradual increase (cos babies don't really use adult items suddenly they gradually are weaned etc) many parents are unaware of the true costs unless they sit and analyse it properly. I'm a bookkeeper by admin trade and I love a good spreadsheet and I've kept old ones from when dd still lived with me and can see the difference in my expenses over the years, I find it quite interesting. I've had similar discussions on occasion with people in real life, a few of whom have been willing to let me see their finances and I've pointed out things they're paying out for which are just for dc which they had forgotten/not counted in their heads.
There are various studies and articles about the fact that parents rarely properly look at this and generally vastly underestimate the cost of raising a child.
Understandably because if you do tot it ALL up to one great big total over the 1st 18 years you'd probably have a bloody heart attack! 
Plus you need to include alllllll the other expenses i and others have listed. You may not spend out on certain things every month or even every year but they still need to be bought/replaced at some point and so the cost is there.
Who started the discussion op? Honestly?
Regarding the unwillingness/inability to stay overnight. That's not uncommon at this age. I was lucky dd was a pretty confident and flexible kid on this kinda thing she was fine staying over at friends and relatives at a fairly young age.
BUT she hated staying at her dads and I didn't learn why until she was a teen. Turns out he was treating her pretty badly when she was there to the point of under feeding her and infantilising approaches to certain things like bath time!
It may also be something to do with the different styles of parenting. She might be babied a little more at home than she knows she would be at her dads.
Not unusual at this age for a child to want a parent to stay with them until they fall asleep which it sounds like her dad isn't happy doing. And are there other children at her mums? Does she share a room there? If so maybe being on her own at night freaks her out a bit?
I shared a room with my sister, when I went away for first brownie camp the focus was on how I'd be staying away from home. I was fine but my sister lost it being left alone at night and ended up "camping out" in my brothers room after the nightmarish first night my parents had as a result.
There's so much to unravel here but honestly neither you nor her dad are exactly coming across well here op