Mum's instinct for survival might be greater than her attachment to the babies unfortunately? I know some species will put themselves over the babies if it's a life or death situation and if she was disturbed she might be too afraid to go back?
The other thing is that babies don't feed that often, do they? So she might feel best leaving them until their next feed, which might be a while away? She might feel safer going back in the evening when it's darker and quieter.
When is the latest it can be left to? Could some sort of trap be set up and monitored from a distance so that if mum goes back to babies they could be trapped together then transported? :?