They don’t really give specifics of what the amount covers they just assess the paying parent and decide on a figure based on their income. You should get the same amount every month/week depending when you arrange payment regardless of whether it’s school holidays. An overnight is literally having them overnight, having them for tea doesn’t count.
If you want additional help with childcare during the school holidays that is something you will have to arrange with him, unless you have a court order stating he is responsible for a percentage of the childcare costs it’ll probably be left up to you. Can you claim any help with childcare?