To ask for the money back?

Exactly what I was about to say 😊

OK to ask, its always been the arrangement, she may have just genuinely forgotten or started doing it, got sidetracked and thought she had.

Don't be embarrassed to remind her. If anything, she might be embarrassed that she forgot. That could be the problem versus her just not paying you. If she's withholding payment, she will likely be irritated rather than embarrassed by a reminder - then you'll know whether or not you want to deal with her and her child another time. Better to find out now than have to deal with the issue in the future.

YANBU, but if you feel awkward about reminding her, maybe you could add it onto the next bill you’ll need to bring up with her? Eg - hi X, this week cost £6, so in total you owe £21 with last weeks cost included’ - just an idea!

YANBU!
Maybe give her until the end of this weekend and message on Monday. Just say that you hadn't noticed the amount going into your account so could she check to see if the £15 payment had left her account yet.
It's easy to forget these things and that'd probably be the reason.
You are a treasure to include your kids' friends in your days out! Have a great summer!

No

Because it is most likely to be true

Either ask again nicely and if they still don't pay, let it go and never ask the child again, they will soon wise up and feel silly. It is not as they don't know. People simply do not "forget" if someone is doing them a big favour, they just don't want to pay and hope you are too nice to ask. Never trust the "I must have forgotten brigade" It is always a lie. Seen it many times over the years, am a granny now. If I know they are like that, I don't ask.

Of course it is possible to lose bank details. It happens to me because my bank only keeps bank details of tge last20 payees. When I pay a new payee, one drops off the end of the list (first in, first out) so the details are lost and cannot be retrieved. If the details were included in the email requesting the £15 payment, the recipient may never have received it or may have deleted it inadvertently.

when tat happens to me , I just hit "resend". I don't get into iany further convo about it. it's easier that way - sometimes messages just go further down the read list and the recipient genuinely doesn't notice. sending "resend" brings them back up to the top.

I'd leave it for now and wait for the next time she asks you to take her DS out

I would say ‘ hi just a little reminder in case you’ve forgotten 🤣’ being a little bit jokey.
It’s easy to forget with busy lives.
A true friend won’t mind you asking😊

“I’m so sorry to ask but I can’t trace the payment. Can you confirm when you sent it?” Then it’s a reminder but making it out to be your error and if she says “yes. I sent it” ask for evidence so you can “chase your bank”

normally works

100% ask. It is highly likely she has just forgotten, but if not, it's no harm to set a line in the sand.

Maybe she doesn't have the £15 ask her if she hasn't then give her a week and ask again meanwhile don't take her kids on anymore days out and tell her you can't afford to pay for them. Be honest about it.

No it’s not at all unreasonable you certainly need to chase the outstanding payment, in the current climate everything counts, none of us have £ to fritter away and if the parents already WFH as you’ve said and that in itself saves £ commuting by train or car plus no buying coffee or lunch out, eat at home. They should be covering the costs of their kids going out with you. You’re saving them ££ on childcare. It just comes across as rude and selfish on their part and you shouldn’t have to ask twice!

Can you check your bank account that the money has gone to the right account as it hasn't come through

Surely you've asked her by now, can you update 😂

Another vote for dropping her a friendly message, the end of term is crazy busy and if she’s usually prompt in paying then it’s almost certain it’s just slipped her mind.

FWIW, my friends and I have a similar setup in that we pay for our own kids on any excursions that cost money and aren’t birthday-related. Sounds perfectly normal and sensible to me.

Perhaps you need a nap :)

Yes, I think @SassyTurtle probably does.

What a negative mindset. I'm glad I'm not her.

How about writing it down? Exceptionally rude not paying someone back money as soon as you can!

If someone was taking my children on days out which help me work in the school holidays, I’d be falling over myself to not Only pay for my child’s lunch but also coffee / ice cream for the OP as well. I wouldn’t let her buy ice creams with no cost returned either.

This

There’s some terribly mean-spirited posters here today. Why is it more fool OP to ‘provide free childcare’? She’s already detailed how it’s mutually beneficial. It’s not a race to summer holiday misery you know. Very good if you OP, hopefully she’s forgotten and needs a reminder. I’m sure she’ll be embarrassed.