The whole reason the Potters went into hiding is because Snape told Dumbledore that Voldemort had already decided it was Harry the prophecy was referring to. The Longbottoms weren't in danger (well, beyond being Order members and all), so they didn't need to go into hiding.
'Well, Severus? What message does Lord Voldemort have for me?
'No - No message - I'm here on my own account!
'I - I come with a warning - no, a request - please -'
'What request could a Death Eater make of me?'
'The - the prophecy... the prediction... Trelawney...'
'Ah, yes,' said Dumbledore. 'How much did you relay to Lord Voldemort?'
'Everything - everything I heard!' said Snape. 'That is why - it is for that reason - he thinks it means Lily Evans!'
'The prophecy does not refer to a woman,' said Dumbledore. 'It spoke of a boy born at the end of July -'
'You know what I mean! He thinks it means her son, he is going to hunt her down - kill them all -'