Thanks for the suggestions. Nelle I like your line. Compromised immune system (enough info without being too specific). Holly I like your suggestion of mentioning another date at the same time, so that it's clear they won't miss out altogether, kwim? DD1's already been talking up the party at school and they're all keen as chips to come. .

I'm going to include it as a separate note to the parents in the same envelope as the invitation addressed to the girls.