To not do this

[WhatTheFluck]

Asked after school nanny today to do a later shift tomorrow, so will finish at 10pm.
Aibu not to do an evening meal for her? She isn't coming straight from another job

I do think asking the nanny is the most important thing. Presumably she has not been working over mealtimes in OP's house so far and it has not arisen, but a conversation needs to be had - because maybe the nanny is a vegan, or allergic to something or other, or on a strict diet. But a friendly 'What about dinner time, shall I leave you money to order pizza, do you want to bring your own, or there's sausages and chips for DC and enough for you as well.'

I don't see why you would have to provide a meal. Does anyone here get meals provided at work
Most people don’t get a meal provided... but they are free to get out during their break and buy something! A nanny wouldn’t be able to do that as you will expect her to stay on-duty the whole time. So a meal should be provided.

You don't need to cook for her but you do need to leave out food so she can cook for your children and herself.