Parents thought I’d babysit an additional child for free… but thinks my rates too expensive when I said otherwise?

Hey all- I previously had a post here but have since taken it down after receiving some good advice. I was hoping there’d be no further updates.

Long story short: Ive been babysitting occasionally for the last 1.5 years when I moved over from Canada. Im in zone 2 London, Islington. I gained lovely clients via words of mouth. A regular family asked me to look after their daughter, but casually dropped less than 24 hrs before the sit that her friend is staying over (who I also babysit separately). The sit is from 17:30 until late.

After advices from here, I told her children from different household will be asked for 70% of my usual rate, which is £10.50 per hour per house. My average rate is £15. I offered to bill the other parents if that would help. The mum was polite but taken aback by the additional rate. She said she didn’t know it “would be so much for children their age” but said she respected I have my own rates. She asked if this is my usual rate for siblings as they’re “practically sisters :) - but see you later! X ”.

I guess she was making it light hearted but I now feel bad since she’s saying she’ll cover for both. I understand £21 per hour is a lot to pay, but it was meant to be divided by two families. I just simply replied no I charge less for siblings, hadn’t really thought to discuss additional charges with you since there were no prior mentions of another child, I hope you understand thanks so much etc.

I don’t know what to do. Im wondering if I should just waive it this time since she didn’t know (albeit presumed free)? Maybe they think evening / bedtime duties should be charged less? I really like this family but I’m also terrible at standing my ground. I’m currently waiting for the parents to come back- the mum was pleasant to me when I saw her off earlier today, no mention of costs etc just catching up. But I can’t help but feel this left a bad taste in her mouth. I almost feel guilty and… greedy?

Please help- should I stick to my guns or not? I feel stressed!