If you are talking about a first cousin to your parents, then the cousin's child and you are second cousins. No removes involved. Your relation to the parents' cousin would be a first cousin once removed.
Foxlee is right, "once removed" means that you go up or down one generation. As you're the same generation as your mum's cousin's child, you will be second cousins.
If you look at the diagram in ethel's post, your mum's cousin is the "first cousin once removed" (the first green box in the middle column) so their child will be the one below this ie 2nd cousin
chazza - I think you're confusing the issue with your second post.. The parents' cousin will remain a first cousin but as each generation goes down on the parents' side, it will be once removed, then twice removed. It is when it goes down a generation on both sides that they become second cousins. Then if one of those cousins has a child, he will be a second cousin once removed. It is quite simple really.
gingersticks - it is fairly obvious that you are keen to 'cur' your parents' cousins' son/daughter but are concerned of the incestuous implications involved. I say just go right for it mooshy mooshy! The blood lines are totally different so it doesn't matter. And don't worry about potential offspring either - they're no more likely to turn out as 'wrong'uns' than the national average. I hope this helps.