travel to Germany with grandson

Yes, you most definitely do, the German passport control are very strict about it. We have taken our two grandsons abroad quite a few times and we always have a letter giving us authority to take them out of the country and to make any decisions regarding their health and welfare. It is signed by both parents and accompanied by a photocopy of the parents passports showing their photos. Last time we were complimented on the letter and told that they wished everyone would supply such comprehensive details!!

I travel with my kids' birth certificates and a copy of my husband's passport as I do not share the same surname with them (my choice, my kids could have had my surname when born)
My kids are biracial and also look more like their father and many people have asked me if I am the nanny - don't get me started with that

I took a letter signed by her parents when i did a similar trip with one of my GDs. I never needed it but to be on the safe side i would have a letter with you.

It is a good idea to be safe.

How exciting, have fun, best check with airline if you are flying.

A letter signed by both parents and perhaps his Birth Certificate. How lovely for both of you. Have a great time !