12-year-old Germans better at English than native speakers

I am coaching a 12-year-old girl in a neighbouring town online. Two Mondays ago, she asked if I could explain a new rule to her about how to use the present to describe the future. 

Basically, you use the present simple to talk about things that go according to a schedule or timetable, e.g. the plane lands at 3 p.m. or the film starts at 10 a.m. When you are talking about arrangements that involve people, then you use the present progressive or continuous (two names for the same thing). So you would say, "I'm having my hair cut tomorrow." That would imply that you've already rung up and made an appointment. You've arranged something with someone else. Another example: "I'm meeting Tom for coffee at 10." Or "I'm flying to London next week."

When I explained the rules to her, the little 12-year-old German girl understood. Imagine my horror when I received similar e-mails from my German mother (who has been in the UK for longer than I've been alive) and my British-born sister telling me about my sister's travel plans for the coming week. Both of them were using phrases such as "I fly/she flies to Munich next Monday." 

How can it possibly be that 12-year-old children in Germany are meant to speak and write better English than people who have spent decades in an English-speaking country or are even so-called native English-speakers? Considering that this girl does not get top marks in English, I shudder [schaudern] to think what marks my mother and sister would get in English. Not good ones, that's for sure.