I have one DD (6) whose father (ExP) has also got his other DD (10) living with him. ExP and I broke up when DD was 1.
ExPDD started living with him 3 years ago, having moved 300 miles down here because her mother was on heroin and an alcoholic. The CPU basically told ExP that she would go into care if he didn't take her. Fair play to ExP as he has stepped up to the mark here (prior to that he'd been basically a crap and uninterested father). He has changed a lot and although we have different parenting ideas and priorities, he's generally a pretty decent father.
ExP doesn't generally make an effort to do things with the DDs though. He has our DD one night a fortnight. His idea of 'doing something' is taking them to the park (fair enough better than some dads and not much money). He doesn't drive but as far as I'm aware, he's never taken his DD on a train to the cinema or just out for a meal etc (she has of course been on a train, but it's always been for general stuff to do - clothes shopping when needed, food shopping etc). However ExPDD doesn't miss out on things as her best friend's mum is generally amazing. BF's mum has ExPDD four nights a week as ExP works nights, ferrys them to different clubs (which in fairness ExP pays for for his DD) and even took ExPDD camping the other weekend.
Anyway, getting to the WWYD bit ...
DD stayed at ExP's last night and I would usually pick her up at midday. However I rang ExP last night and asked if it would be ok to pick her up about 10ish as I thought I would take advantage of the good weather and take her to an adventureish smallish theme park around here. No problem he said. I then texted him this morning and asked if his DD would like to come with us (for a change and for her to do something) but I also said it was £12.50 to get in which I couldn't afford. He text back saying it was a nice idea, but he can't afford it.
This is where our parenting ideas are different (and also one of the reasons we broke up). He told me he was going to the pub today to watch his footie team. He was also at the pub on Friday (I saw him coming out of there whilst driving past) and he said as it was a nice day yesterday when I dropped DD off, they may be stopping for a quick pint and ice cream. So he'll have spent far more than that on drink over the course of the weekend - and does every weekend. One of the reasons we broke up is that he didn't care if we didn't have food on the table as long as he had money to get a pint or four.
ExP has no qualms of taking advantage of people and almost doesn't realise when he is (or gives a good impression of not realising 
) and that's why I put the price in the text and said I couldn't afford it otherwise he would have just assumed it was gratis and would never offer to pay (has done that before when I took his DD to the cinema - despite me putting the price in the text)
So do I take his DD anyway just for a nice change for her (technically I could afford it, it wouldn't 'break the bank') or do I not.