Do I need parents permission letter to take my 5 year old niece on holiday to Spain?
Absolutely, you need a permission letter from the custodial parent. If they have shared custody, then you need a letter from both of them. In addition, you need a power of attorney drawn up so that you can provide any emergency care your niece might need while you are gone. Going international is a big deal. If your guest is not your own child, then you need legal authorization. Get on it.