Newcomer Seeking Advice for my son please

Hi I am writing to seek advice on Hand, Foot and Mouth Disease (not to be confused with Foot and Mouth Disease) which is apparently a bit like the chicken pox virus

My 3 year old son had a coarctation of the aorta repair at 8 days old and has a bicuspid aortic valve and aortic root dilatation that is monitored every year and is currently 3.5 SD from the norm whatever that means in real terms! We have been told that there is a risk of endocarditis but the UK guidelines were changed by NICE last year such that antibiotics are no longer required for dental work etc.

On Thursday, we had a letter from the school they both attend saying there is hand foot and mouth disease going around and it is viral, highly contagious and causes lesions in the mouth, hands and feet. On Friday, my 6 year old son said that it had started in his class and that one of the girls was in the class and they were told by the teacher not to hold her hand. I know at least 4 children have it already. I found out that the school said its no point staying off as they're all going to get it anyway. I looked into the health advisory website and they let children into school as long as they are not feeling actively unwell although you are contagious for 10 days.

I am panicking as I didn't think about the risk from lesions until this evening for my other son who is in the nursery at the school. My 6 year old has had runny nose and high temp since Friday night which is the precursor to the virus although no lesions yet.

I would appreciate any advice that anyone may have.

Thank you.