My partner & I are just back from a mini-cruise and stay in Amsterdam taking his daughter and her sister (8 & 11 yr old). Fortunately their mum prepared letters confirming her consent to the trip as we were asked for evidence at every passport control both in the UK and on arrival in Holland. We plan on taking them further a field next year and will certainly make sure we have consent letters with us. We also had a bit of a problem getting insurance as we don’t all live at the same address, or have the same surname, we managed to get a family annual policy from Insureandgo.