Well, I have to say that 'X' was a difficult one and I'm very surprised that I managed to come up with anything at all but here it is.
X shall stand for playmates ten,
V for five stout, stalwart men.
I for one, as I'm alive,
C for a hundred and...
D for five (hundred).
M for a thousand soldiers true,
Now all these figures I've told to you.
This little nursery rhyme was used to teach children about Roman Numerals and I must admit, I have now learned it as I think it's really useful but unfortunately I can find no reference as to when it was first published or to any of the history behind it. It's a sweet little rhyme anyway.