What is the meaning of "Already wet, aren’t they? Little squirts!"?

Solution 1:

First, some definitions:

• In UK slang, wet means weak, wimpy, pathetic, spineless. This sort of thing is (or was) commonly heard in schools:

  Don't be so wet!, or

  I can't believe you won't climb the wall with us. Girls are so wet!

• A little squirt is also slang: it's a mildly insulting term for a small, insignificant person (usually a child).

So, the quote: "Already wet, aren't they?" is playing on the two meanings of "wet" - they're already wet (spineless), so it doesn't matter if we get them wet (with water). (This is more likely than the pure literal interpretation of "wet", coming immediately after the stereotypically girly "wet" behaviour of screaming and running away - Peeves's comment is in direct response to this, hence the aren't they question tag.) And similarly the little squirts also refers to the girls, in a similar act of insulting wordplay (squirt=small person; squirt=jet of water).

Solution 2:

It seems nobody has bothered to post the most obvious, and probably most likely, interpretation: already wet refers to the fact that it's pouring rain outside, so the girls — who just came from outside — are probably sodden; and little squirts is a mildly disparaging term for children, as well as being a slight play on words because squirt as a verb means to spray water.

(In case it's not obvious, both phrases refer to the girls, and Peeves is kind of talking to himself — narrating his own actions, as it were.)

There's really nothing all that complicated going on in this passage, language-wise. To call the peeing interpretation "far-fetched" is a massive understatement.