To be expected to run around after another's child?

You’re a pushover.

Don’t make excuses or tell lies or apologise or explain. Pre-empt her, it’s much easier than being caught on the back foot. Text her and say: “Hi CF, hope you’re well. Just wanted to say - you’ve been asking me to bring CF DD home a lot lately, please don’t do this anymore as I won’t be doing it, so you need to make your own arrangements.” CFs need telling straight because they have a brass neck!

Your daughter is like this because YOU are.

I agree. You are teaching her that she doesn't matter.

Your daughter is like this because YOU are.

Do you not see the correlation between your behaviour and hers?

She naively thinks this girl is her friend because they share a hobby. You naively think (as per your own words) -

now I have to drive 4 miles out my way to drop them back. Err.. you don't.
How do I get to be there, watch and collect my child after having declined to drop off, watch and collect hers every week? Again, you don't.
I want to say... Ruddy well say it then

Please stop. You are teaching your child to be a weak people pleaser. Teach her through your own actions that you are not going to be bullied. The lessons she learns from you will set her up for life. For good or bad.

Ultimately you have two options:

1. Say no
2. Carry on being a doormat

There is no magic 3rd option that'll work like a Get Out of Jail Free card. But saying no gets easier the more you practise it - so there's that

This is kinda your own fault. You can just say, "no I can't, sorry." I used to let people walk all over me because I'm a stay at home mum and they work. But I grew a back bone and always say, "no I can't, I'm sorry." If they ask why I say, I'm not sure who will be taking mine yet/I'm going to my mums straight afterwards/I don't feel great etc. Any messages from them get ignored until hours later. As long as you say no nicely, there's no fall out.

@Blondeandbeautifullol .. and then she has tge audacity to expect me to facilitate it! … and you DO facilitate it! Thats your problem!

Why can't you just say no. There's no way I'd be doing this, none of you even like each other!

You can just say you aren’t dropping off anymore. However a large portion of your OP is purely your own thoughts about others motives and are not necessarily based on fact.

I’m also a people pleaser and would struggle with this. But I think I would just give a white lie. Hi alpha mum, really sorry but I’m not going to be able to give your DD lifts anymore to rowing. We have family things on straight after from now on and won’t be coming straight home because of it. Thought id give you the head’s up”

In fact @Blondeandbeautifullol, if you really want to make sure you're heading this off at the pass, contact the coach today and say, "Just as a heads up, I will not be taking Jemima home after practice tomorrow or at any point in the future. Jemima's mum and I have no arrangement about this. She is just telling me, or if I don't reply, telling you, that this is what is happening. Can you please contact her and say she needs to pick Jemima up at the end of practice tomorrow, and every week, because there is no other option?"

Can completely understand why you are facilitating this other child but you need to start changing this situation. Let the rowing organisation know that you are not responsible for this other child and will not be in the future. Tell the mother as well and then BLOCK her.
Then let the school know exactly what is going on so that they can support your daughter. Your daughter obviously has an ability that she is developing through additional support.
She needs to be able to continue this in peace and without this frenemy around.

It isn’t your responsibility to provide an opportunity for this pair to ride roughshod over your child.
Because this is what is happening.
Stop the lifts, “I’m unable to help” and if she asks via the coach, say the same thing, you don’t have to give an explanation why. Like other posters have said, the kid won’t be stranded. Her mother will either have to get off her arse herself, send a taxi for her or the coach will have to take her. I bet you the kid suddenly stops the activity once the mother is the one to be inconvenienced and not you.

All of this is just making the whole thing more painful IMO.

She shouldn't give this other mum explanations or arguments which she will then try to counter.

She needs to just say no and block her number, and tell the coach to stop passing on messages.

Can you just say "I won't be able to pick up and drop off from now on" or even brush her off a few times by saying you're not sure you'll be going and then she can't rely on you.

if she texts you to bring them back while you're there say sorry I've got to do X on the way home and say you really have to get off and if she's late she'll have the coach waiting and I guarantee she won't do it again.

Those parents are enableling their child to learn unhealthy competitity and be a user.
Don't help them.

I honestly don't think OP should engage in any conversation with the mum at all.

The OP is conflict averse and the mum is a CF.

OP just needs to block her number and ask the coach not to pass on any more messages, explaining that she's not being a helpful go between, but facilitating cheeky fuckery.

You can't, you've got to go look after your elderly parent/ grandparent/ visit someone in hospital/collect your partner from work. You're busy from now on.

This. But as this lady is a cheeky one I would be prepared for push back so have iron clad no at the ready.

This. Just say it isn't convenient. None of her business why not - it just isn't.

If she pushes tell her you don't want the responsibility. If she finds it difficult to balance her career and her child's activity that is something she should have considered. Perhaps she could pay a childminder to collect her child.

Don't lie and make up relatives to visit etc. If necessary, I would tell her outright that the girls don't get on and it is a very stressful atmosphere when they are together and you aren't prepared to do it. (I will say that at one time I would have felt very guilty about something like this - I used to be a pushover - but now I'd just say "Sorry. It's not convenient."

Also voted YABU because you should say NO! Rather than silently seething and posting here. If she wants her dd to do an activity, she needs to arrange transport herself.

Absolutely this! ⬆️ No is a complete sentence. Don't put yourself, & more importantly, your daughter, through this shit.

Time for a gloves off, honest conversation -firstly, with the CF mum & then your daughter - about breaking away from her bully & dealing with any potential fallout.

Step 1: Block the mother's number.

Step 2: Speak to the coach.

"I'm in a bit of an awkward situation and need your help to resolve it. Jemima's mum and I have never come to any kind of arrangement about giving lifts. We have never even discussed it. I don't want to do it because we live nowhere near each other, so dropping Jemima home is a detour I could really do without, and there's nothing in it for me because she's never once offered to give Zoe a lift in return. She has just made a completely unilateral decision that I can take Jemima home after practice. She will message me during the practice to tell me - not ask me - that Jemima needs a lift home, and if I don't reply she will message you and ask you to tell me. I feel unable to refuse because it would mean leaving a child with no lift home, through no fault of her own. I've now had to block Jemima's mum's number, but the next time she messages you, can you please not pass the message on to me but tell her she needs to come and pick her own daughter up? I'm so sorry Jemima's mum has involved you in this situation."