Nichola thanks for drawing out this important issue, it needs to be stopped.
You've got to be cruel to be kind and get this child labour exploitation stopped. By firstly politely declining the service, advising the restaurant or bar owner that they are allowing the exploitation of children by allowing these children onto their premises, and they should stop it. Telling other people about this, and encourage them to stop giving them money. If necessary then advise the Polis/Zabatta or whoever and ask them to try to stop it. When the money stops, so will this particular exploitation of vulnerable children.
And for those of you too tempted to give the money across because it is all so sad, then salve your conscious by making a donation to the many charities that operate in the UK and across the world to eradicate the exploitation of child slavery.
BTW Nicohla that's the second time this week I have praised your contribution, you must owe me a beer by now