WD40 is fine for the job, used it hundreds of times in different situations.
Mike,
its a wet non-volatile solvent I'm Sorry don't know where your going with that, you've answered your own question.
The guy asked what could take out crayon and WD40 will with no problem at all, he said the crayons were wax,which suggests they are not the water soluble variety,so just using water will have no effect.
Most crayons contain oil in one form or another so its going to need some help in being removed and I suggested an alternative to the strong solvents you carry on your van.
If you carry the battery of solutions and solvents on your van as suggested on here then you could try them all, I would just pick up a tin of WD40 AND HAVE DONE WITH IT.
Apply WD40 it will soften it and then you can either scrape it away or extract it.Its best to extract as much as possible to avoid bioaccumulation.
It is biodegradable and will evaporate away leaving no determinable residue, it is not toxic, unless you fancy a pint, it is cheaper if used for not only stain removal but also as a tool, you should all have a tin on your vans.
I said 'in the right circumstances' of course if the householder has an aversion to the slight vapor given off then don't use it.Using solvent cleaners in the home would give of more odour anyway.
By the way you can flush it down the sink if you want it is marine toxic free.

And its good for athritis as well.

Anyway good luck with it chap, I am sure you will do a great job.