Tolkien forever

EXACTLY. I honestly think a lot of people confuse him and C.S. Lewis.

It's literally a children's book. It won a best juvenile fiction of the year award for 1938 and had a Carnegie Medal nomination.

The Hobbit can certainly be considered far more a children's book than The Lord of the Rings, but it isn't exclusively for children, which is what is implied. It was more of an introductory book for the LotR world which he was already working on but not nearly finished with (which is why there were a number of edits made between the first and second printings). But to say "then write a children's book in it" is a vast over simplification of not only the work but the weight of the world created there, to the point of being vaguely insulting to all.

I'm pretty sure it was meant to be humorous, not insulting.

But its TOLKIEN.