IM(very limited, second hand)E, not normal for them to play sick well enough to make you wonder. I certainly wouldn't rule out complicated mind-body connections confusing the picture, though. What's up? Something really worrying or just "do I keep this child home from nursery" stuff? If you still suspect playing sick, I'd set up a system that if you're too ill to [whatever] you stay quietly in bed and don't get to do exciting stuff. Hopefully this is a bad result in the case of playing sick, but a good one in the case the child is actually feeling sick for whatever reason.
