Hi. I remember my son having slapped cheek and I'd never heard of it before the Doctor told me. There are loads of little viruses that kids get and not all have names.He was a bit rashy, pink-cheeked, bit of a temerature-if I remember right. I think it is very catching but I don't think it can harm pregnancy- but maybe you ought to check that its not german measles, or just ring the Doctor to be sure.
Yes, I'd keep him off nursery for a few days,the other Mums wont like you otherwise!