To ask her to pay???

[UnicornsAreReal666]

Quick backstory just so you all dont think I'm being a bitch

DC birthday party coming up, for the past 3 years my DF has not turned up to my DC birthday, (with their DC of course) literally cancelled on the day or an hour before, every time, either her DC refused to leave the house or just couldn't be bothered, this is a known thing for her.
I Asked DF to let me know if she will be coming to DC party as I was paying for it v soon, and didn't want to get them a place their not going to turn up or cancel, by this time id asked her 4 times+ with no specific answer, so as you do I payed for the party, but not place for her DC.

Today a week after I've payed up announces that she may make it.

The AIBU part is, AIBU to have her pay for her own DC? She did offer I just don't want to be a bitch because I feel like one haha.
Reasons.

1. Incase she doesn't turn up/cancel and again in out of pocket.
2. Ive spent the birthday budget now as its next week! So even if I wanted to I haven't got the spare money to pay for another place.
3. I asked her 4 times maybe more as I see her almost everyday.

When she asks unreasonable favours I'd laugh in her face and say you don't have enough time, you have your own family/home to look after. Pretend it's a joke. If her interest wains when you stop helping her out she wasn't ever a friend.

I can see you're going to ask her to pay for herself. Good. Now stop walking her kids to school!

How did it go @UnicornsAreReal666? What did you decide to do in the end?