Hiya,
My DS had this very badly when he was 4. Giant unpainful boils on the backs of his knees and insides of his arms. They were pretty spectacular and it was all i could do not to squeeze them
!
I tried everything at first. Silver solutions, apple cider vinegar topically, antibacterial cream, homeopathy etc. After about 6 months it was clear none of it was helping and some of it actually hurt him. As they genuinely didn't bother him I gave up. He still did his swim lessons, but in an all-in-one sun suit. He wore light cotton pyjamas with long sleeves and trousers (that looked like day clothes) all summer as this didn't irritate the spots.
His sister did eventually get it too despite not sharing towels. She only got about 10 spots in all, mainly on her neck. I think there was an element of inevitability about her getting it as they are very close and are often in contact with each other bouncing on the trampoline etc.
After about a year, one day I suddenly realised he had no new spots and within a week they had totally vanished. Unfortunately, or maybe reassuringly, this is just an infection that needs to clear of it's own accord and from what I've read, there is not much that can be done about it.
It's pretty spectacular though at its' worst, so my sympathies! Hope that helps.