To not invite their kids to our wedding

Completely agree with this too.
You can invite your dcs' friends round to play without taking them out to expensive places.

But, if you did buy a ticket for something, then you should have insisted on being paid for it, if that was the understanding on which you took the child.
Food and drinks is different, as they might not generally spend all that additional money on top of admission prices when they go on trips.

only a handful of kids, that are family or the kids of guests who are friends with ours.

You've stated what your 'rule' is, and, following this, you need to invite the whole family.
I'm totally on board with not inviting dc, or not inviting dc other than nephews and nieces, but you can't invite your other friends' dc and not hers.

First of all just do play dates that don’t include additional expenses, just have the girls at your house if you want them to play together.

Secondly of course you can just invite the adults, even if you’re inviting some children doesn’t mean you have to invite everyone children. We had some kids at our wedding but only very close families kids or those that were travelling across the country to attend the wedding so would have had needed someone to have their kids for the weekend rather than just a day/night. You can invite whoever you want to your wedding for any reason you like, ignore those that say if you’re having some kids there you have to have everyone’s kids there

TBH I really think you could have managed this better from the start.

Not paying for everyone's food.
Not continuing to organise playdates that weren't reciprocated.
Reminding her to pay you back.

You've avoided some pretty basic communication and problem solving and want to jump straight to the nuclear option which will definitely cause issues for your DP's friendship.

If you don’t want extra children at your wedding, then that’s your choice and it’s an understandable way of making a saving. However, some people get funny about it and they might not go or else one of them might not go. How would you feel is only the mum went?

Don’t not extend the invitation to the children out of revenge or spite though. A wedding isn’t the place for that kind of behaviour.

So the mun is tight and shit so the solution is to invite her and not her daughter??

Why would you invite them to your wedding at all? I don’t get it.

Why is this all about her?

She's not the child's only parent is she? And considering the child's other parent is your DP's very close friend, why didn't you arrange the playdates/money etc with him?

I think you are getting a hard time on here OP. Had you skipped the first part of your post where you had described the mum's CF behaviour and just said 'it's a small wedding and the have three small kids which means three extra places, can I just invite the parents' Everyone would be saying, it's your wedding, your choice.

I think you can invite who you want.

It sounds really petty. Is DPs friend not the father of this kid? Is there a reason you don’t have anything against him?

Why are you upset that the mother doesn’t reciprocate playdates but have no animosity towards the father?

When they turn up and there's other couples there with kids, it's going to be obvious they're the exception. How does your partner feel about it? Surely his opinion matters too, or does he have to do as you decide because it's your wedding?

Are other kids going? If they are, you can't then exclude her kids.
Just stop arranging anything with her.

I wouldn't invite the kids, and I wouldn't feel bad about it or worry if that meant they couldn't come. They don't exactly sound like great friends.

I think for a wedding it's etiquette that either children are included or they're not. It's unusual to invite some people's children and not others unless it's family.
If you're inviting other children, I would include your friend's. Put aside any penny pinching history for the day.

Also you should expect ‘oh we don’t have babysitters’ message

Then reply, sorry yo hear that, I’ll mark you down as a decline. Thanks for letting me know.

Id also invite the friend plus one on the basis your DH likes this man.

No need to trouble yourself with the kids. Your DD will have plenty of othered to play with.

What does your DH say?

Oh I see there’s lots of dcs, not just the girl your dd is friendly with.

I think it would be fine to just give a couple invite, essentially you are inviting DHs friend and his “plus one” but not the whole family.

I also would make a decision I wouldn’t invite their dd out for the day until they had hosted dd, even just a play date at their house. Invite other friends of dds from school etc.

I'd be worried about the family and wondering if they're not as well off as you...

How about your DP talks to his friend and asks to be paid back? If they are that close I don't see the issue.

Also, doesn't need to be paid play dates, can just be the girls hanging out in garden, going to park, watching a movie etc. I used to arrange elaborate play dates for my daughter and her friends with farm trips, trampoline park, soft play etc. But then observed that they seemed to have the most fun doing their own thing

Your wedding is next year, no more invites for her dd and see how you feel then , it's a bit dramatic

Do you really know their financial situation?

I say this because when my kids were younger we were so skint that we just couldn’t afford any kind of paid days out… but outwardly no one knew that

Excluding the DD because her Mum didn't pay as fast as you'd like is a shit move. Do you know their financial situation?