Oh no!
My DDs have just had this in Feb & March this year. And what most say is pretty right - it took 5 days for DD's spots to stop appearing and then another 2 days or so for them to completely scab over. He will have lots of scabs by the time you go - my DDs still had scabs (partic the baby) 5-6 weeks later in places!
Defo correct about Medised and I have to be honest and say I'm surprised any preschool would administer a medicine as strong as Medised to little ones - I would have thought Calpol would be the norm? Medised is very strong stuff (and yes I have used it on my DD1 before the rules were changed but would be cautious now) - it can easily knock out children/almost drug them and cause hallucinations - one of the reasons it was stopped for under 6s....
Also, just a watch out, if Macklyn does get it you are v limited to what you can give him - he can only have Calpol (if over 3 months) on a regular basis I believe - no piriton (antihistamine) under 1 years so he may be pretty off with it tbh. My DD2 was 9 months when she got it (exactly 2 weeks after DD1) and tbh she was v uncomfortable when she itched - we had nights of hardly any sleep as she just couldn't sleep with being uncomfortable

In the end the out of hours Doctor said to give her a half dose of piriton but only because she is a big baby (more like 15/16 months at the time!).
I don't mean to scare you Jon, CP is really not always that bad, but I've just had a friend whose DD was hospitalised with it (she is 2.5 yrs) - her spots got infected and it turned v v nasty indeed. My DDs both also got infected spots - had to have antibiotic cream from GP to help - fortunately I caught it quickly.
They are looking to possibly vaccinate against CP in this country - maybe try your HV and see if they could vaccinate Macklyn - I know you can pay and have this done privately but I believe it doesn't give 100% protection (Bit like single MMR jabs)... may lessen any CP if he does get it.
As a Mum, I was honestly never too worried about CP but having had both my DDs get it earlier this year and having experienced what we went through, I honestly would not have put my child through a trip abroad at the time unless they were well over it - I found day 4/5 the worst with both girls....
What awful timing for you - I guess you'll just have to see how it goes

I would have thought you could cancel on your insurance though if need be. Not what you want to do I'm sure, but if Macklyn does get it when you are away and he turns out to be quite ill..... You really can never tell.
PS - I was told by my GP and lots of other people that DD2 probably wouldn't get it as she was breastfed and under 1's tend not to get it - she still got it and quite badly (was told it would be mild!)....
Good luck with it all! It's an awful decision to make!
