Most anime is made for children, at least if you count episodes instead of series. Yet adults enjoy much of it. So where is the borderline?
Erin has recieved a lot of attention, which is not surprising for an NHK show with the associated budget. It is really well made, and I believe the claims of a rewarding story. But I looked into two episodes and couldn't finish one.
It starts with loads of painstakingly slow exposition, to make sure everybody understands the world. There is a trope that whenever two characters talk about something that should be obvious to them but not the viewer, it is bad writing. Here the problem is rather that the target market was young kids.
A similar show is Tweeny Witches. It ran on the same channel for the same audience, but I loved it. I'd have to think about what makes them different, but for me the childishness line is somewhere in between those shows.