The fact that it's happening to both of them would indicate it's not just something that one of them in allergic too, but have you considered heat rash?
Cat fleas can hatch out this time of year though and up to three years after the cat has gone (sorry to be the bearer of bad news). But they won't survive on human blood for long so I don't think that can be it if they've had them for months.
You can put sudocrem on them btw, that seems to help whenever DD is bitten by nasties and if it's bacterial that will help with that too.