I would think you would have to look at what you want them to be able to buy with the pocket money. Do they have to get their own phone credit, magazines, cinema tickets, what ever they usually do so maybe 15 euro- this might depend on where you live as cinema and the likes are dearer in the bigger cities. Also look at how much you can afford.
You could use this as a time to educate your child about budgeting. You could give them a decent amount say 40 euro, pending you can afford it, and get them to save some so they can by their own non essential clothing, also put some into long term savings, buying their own tickets for things.
You have to bear in mind also that your child is at an age where they may start drinking or smoking, so having the money will make these things more accessible. I think it really depends on the child.