How much should I pay for this arrangement?

[BoatyMcboatface12]

We want to raise our child (currently 3 months old) trilingual using the OPOL method. My husband speaks 2 languages, I speak 1. He speaks to our daughter 100% in one language, I speak to her 100% in English. We have a friend who is currently unemployed but who speaks this third language. We were thinking to ask if she'd want to come to ours a few hours a week, look after the baby (but I'll be there in case she needs me), and basically just speak to her in this language.

This isn't quite childminding or babysitting as I'll be on hand to help if needed. It's something between a "mother's help" and an informal language tutor. What kind of pay would be fair for this sort of work? It's in London and we'd probably need to cover her transport too.

You need to think this through much more.
Will your child be exposed to this 3rd language outside of her babysitter?
what will you be doing when she’s there?
does your friend even want to do this? If she does what does she want for doing it? I’d never want to be employed by a friend, and I doubt most people would want it.

few hours per week - in one block? so difficult for her to find other work on that day. And she has a specialist skill. £20 per hour?

I'd pay whatever the going rate for a language tutor is and remove any childcare aspect.

I think you're wasting your time (and money) tbh. A few hours a week isn't going to be enough to achieve any kind of fluency. What will happen when your friend finds a job? Will she be easily replaced with another babysitter who speaks this language? If not, then there is little point in a few hours a week for a few months.

Does your DH speak this language?

I’d say don’t do it, mixing business with friends is never a good mix.

Two languages consistently is much better than 2 with an additional third as hoc

Waste of time for a few hours a week. Stick to two.