She is a mere child = she is only a child.
The other three types of mere, however, are out there and fully visible.
The first sort of mere is a lake or marsh, and probably not a very cheerful one. (But then marshes aren't often cheerful.) I associate this sort of mere with Tennyson at his gloomiest:
Long stood Sir Bedivere
Revolving many memories, till the hull
Look'd one black dot against the verge of dawn,
And on the mere the wailing died away.*
The second mere is a boundary or boundary marker:
tri-state boundary marker, USA. Photo by Cohee
and the third is a flat club usually found in New Zealand:
I suppose this is a grim thing, too, but it does at least sound cheerful: you say it merry, pretty much.
The boundary marker seems the easiest to spot, as well as being the most fun, so I think I'll go off and find one of those...
...would a garden fence count?
Spot the Frippet: mere. The lake word comes from the Old English mere, meaning sea. The boundary word comes from the other Old English word gemǣre. The weapon word is, of course Māori.
*Those are the last lines of Morte d'Arthur.