Child travelling abroad and money

Most schools will let you know how much money to give for the trip.

Most of the time they will also tell how, cash, card or other methods.

How much depends on what the kids have to pay, how much you allow as extra money and you will likely adjust the method to the amount.

If it is just a small amount, for some souvenirs and the odd snack, cash in euros is still the easiest. Unless the kid already has a card that will work in Paris.
If the kids have to pay for lunch cash and/or a prepaid card.

I would not give a teen a credit card, certainly not without a limit on it.
Besides, for the small amounts I expect a teen on a school trip to need, credit cards may not be accepted. (Although the comments on this and other answers indicate that in France credit cards and debit/bank cards on the Mastercard and Visa systems are all accepted in all places that accept cards, but there might be a minimum amount depending on the place.)

If your kid already has a smart way to pay, like a phone or watch, that might work.

If the school tells to bring cash and each kid has to arrange their own, see if you can get other parents to work together, saving effort and money on the transaction. Or ask around whether friends or family members still have euros and you can buy them for the market price without having to pay a bank, good for both parties. (Internet does give good values, just search pound to euro or dollar to euro or whatever.)

Some school trips have a teacher collect the money at the start and handing it out once a day, or more often, on request. This basically works with cash but some schools may accept money in a bank and hand it out in cash, so if the kids need to have quite a bit of money that might be worth asking.

In most european countries, both cards and cash are widely accepted. However, "cards" means bank cards, not credit cards. Many smaller shops do not accept credit cards. So unless you can provide a european bank card (Girocard, EC-Card), cash is your best option. France is not one of the few countries where cash is going away, though the Corona epidemic is accelerating the trend towards cash-less payments all over Europe. On the other hand, street vendors and some small shops may not accept cards at all. In summary, that you cannot pay with a card is still more likely than that you cannot pay in cash.

Cash also has the advantage that it is more tangible. Children often do not (yet) understand money well enough to deal with it entirely in the abstract. A €10 bill is still a lot more real than a €10 card transaction. It will enable your kid to control its spending better.

You don't say how long the trip will be. If more than a week, I would consider also including a "reserve fund", which in case of emergency or unexpected expenses can be turned into cash. This can be the old Traveler's Cheque or a debit card or something else that takes one additional step to make it less likely that it'll be cashed to buy some cookies (or croissants).

So I have been to a few school trips in foreign countries (Tho only one with a foreign currency) and my school always advised our parents to give us cash. Its easier to spot how much one has left and many smaller stores or bakeries don't accept credit card in Europe.

For the amount: I was given 100 pounds for 6 1/2 days in Great Britain about 7 years ago.

Check with the school. They will likely provide guidance on this nearer the time anyway, along with information about what your child needs to bring with them (and what they should not - e.g. expensive electronics or jewellery).

Many school trips recommend cash and operate a bank system. Before departing, each child gives the teacher the majority of their foreign currency and the teacher looks after it for the duration of the trip (often along with their passports). At regular intervals there will be an opportunity for each student to withdraw cash from the teacher's bank. Typically this would be at breakfast/dinner time, or immediately before a shopping trip.

Source: I have accompanied trips with several UK schools.