There's something to be said about the homography between trees (NL: rhymes with trace) and trees (EN: rhymes with peas) juxtaposed with "aap" in this joke... but this is not the forum for eth(n)ical debates.

This isn't poetry - so it doesn't need to rhyme, per phil's suggestion - it just needs to be neologized [is that a word?], split and concatenated.

Nonetheless, however derogatory I find this joke to be (at several points along the spectrum), I'd opt for traipse, because I do not gather the joker is speaking positively of either the mother or father, yet is saying somehow that the person in question is a "catch" (fig.) or literally a trap (i.e. trouble waiting to happen). Doing so will mean the punchline can be preserved as well.

The solution in my estimation should be along the lines of:

traap => trap
luie aap => lazy chap
trees => traipse (or trapes)

traipse + chap = trap

This is the closest you're going to get I think - at least taking De Bo and Samyn's West Flemish Idioticon into consideration.

*If* the person being told the joke was the one being blasted, then I might switch it up deliberately as a 'yo momma' joke. That's not really the case here, but I'm going to put it here anyway for fun (see "Example sentence(s)") because it makes for a better joke IMO. Just presume the following for the 2nd sample sentence:

traap ~> trapes
luie aap => lazy ape
trees ~> trap


Otherwise, if you go with "catch" for "traap", other connotations behind "trees" in Flanders may be equally if not more offensive in English, but the literary elements relating trees and apes (trees & aap) are preserved too.

traap => catch
een luie aap => a p*ssy (sl. as in lazy coward ~ lit. cat)
trees => dumb b*tch (sl. malicious woman ~ lit. female dog)

Cat relates to dog, Ps to Bs (in terms of female genitalia), and you cross a cat and a b*tch and you get a catch.

In other words, use your better judgment. I don't like this joke, probably because the last solution is the only one that holds enough irony and interlinear relationships, but there are many ways to go about it -- specifically depending on the audience -- it seems like she wanted to draw the person in by giving them a compliment, then flipping it on them talking badly about the person's parents to go in for the punchline.

Curious what my colleagues think about this line of reasoning.

--------------------------------------------------
Note added at 20 hrs (2014-06-21 19:41:59 GMT)
--------------------------------------------------

Meant to add: http://www.vlaamswoordenboek.be/definities/term/treesebees

My wife's cat was called trees; it is a name with ironic-bordering-on-derogatory connotations of 'aardappelmeisjes' that you could equate to the Essexy 'tracey' or something like 'chantal' in the 2000s.

so you'd get something like tracy/ape = trape, not funny in English either imho