My 5 month got it at 3 months old - and he barely even noticed. 20 spots max. We kept him in his baby grows with the mits over to prevent him from scratching but as far as we could tell, they weren’t bothering him. Doctors said it was good for him to catch it whilst he was young
My 4 month old got chicken pox a couple of weeks ago, our GP said it wasn’t even worth an appointment. Just apply calamine lotion x