I think your ex is letting you off fairly lightly to be honest. Your only giving her 250 baht a day - which as you say would just about cover the items above. 250 baht doesn't get much in 7-11 now.
That means she is covering all his living expenses - accomodation/rent, electric/water, clothing, toys, healthcare, transport. I'm sure they don't sit at home all weekend so she's taking him to the malls or parks. Something like harborland playground is 4-500 baht for a couple of hours - follow that with a visit to McDonalds maybe and another few hundred baht.
My daughter is 9 now but when she was 5 or 6 she would often get sick - fever or bronchitis etc. Trip to the clinic easily costs 1000 baht + with medicine. I think your ex is covering many costs your not even thinking about.