Thanks for your email. I hope I can help you.
I am aware of very strict regulations regarding this type of arrangement when travelling to South Africa and can prevent travel if you do not have the appropriate documents and sworn declarations. But I am not aware of such requirements within Europe.
However, this does not mean that the airline staff will not check. They may question you and if they do then it is better to go prepared.
My suggestion would be to prepare a letter of consent signed by both parents. In it you should include the flight details and set out by reference to any documents your relationship to the children. You can then attach copies of the documents (birth certificate, passports, marriage certificate etc. I do not think you should have to get copies of the documents certified by a solicitor, the parents could just write: I certify that this is a true copy of the original, then sign and date) to the letter of consent.
Whilst this may not be necessary it is better to go prepared. Better to be safe than sorry.
Do let me know if you have any further questions or queries. I would be happy to assist you further.
If not, please can I ask you to take a moment to accept/rate my reply. Otherwise I will not get paid for the time I have taken to consider and type my reply. Thanks.
P.s hope you enjoy your holiday.