I happen to be in Germany right now!
I knew from previous international travel my U.S. provider woudn’t support an international plan. I ended up getting this sim car —>
Orange Holiday - Europe Pre-paid SIM. It works in a huge list of countries, and I have no complaints. I did have to order it from Amazon, because the Orange website does direct sales for eSIMs, but in this case via Amazon is not a scam.
The card does say its only good for 14 days, but that restriction is lifted once you register the card.
If your kid’s phone supports an eSIM you also have that option, direct from the Orange website.