Budgeting would be great but I think you could branch out and do it more about "adult" money in general (e.g. tax, NI, pensions, savings).
So you could start off with an opening question for group discussion:
"What is tax and why do we pay it?"
Then after they've fed back and you've actually told them what tax is, you could give them a pen and paper and ask them "What percentage of our wages do we pay into tax?". Get them to be silent, not confer with neighbours and write down what they think. Then everyone who said under 10% put their hands up, between 10%-20%, 20%-30% etc.
Then you can tell them how much ta we pay on certain amounts of earnings. Then give them a calculator and 5-6 examples of different earners and they have to work out the tax each person will pay.
Obviously prizes for the most correct answers.
Then you could do similar with pensions.
Then the afternoon could be about budgeting and saving.
I say this because I started work at 18 and I had literally no clue about tax or NI or pensions. I mean, zero clue. I was a very worldly-wise and academically clever young woman but I had no idea at all.