Training her out of it will only work if you are utterly consistent - and if there is any doubt that your son is as consistent as you or your OH you will need a harness for him to use so he doesn't undo your work.
I'll talk you through the method we have started using at puppy classes - it seems to be pretty effective...
Step 1. Have a clicker and a handful of treats.... attach lead to collar - lead should be in a nice "J" shape (so completely loose, hanging down).
Decide which side you want Roxy to walk on (eventually) - if you want her on your left, hold everything in your right hand (lead over wrist, clicker and handful of treats in hand) - use your left hand to retrieve one treat at a time after clicking.
Have Roxy infront of you, step backwards - if she chooses to follow, click and treat.... repeat many many times.
Step 2. Once she has the hang of following you, do 2-3 steps backwards (clicking and treating), then turn, so that Roxy is in a heel position on your left - click and treat. Then return to being infront of Roxy and repeat this stage many many times.
Step 3. Short walks on lead in a low distraction environment - Literally just about 10 metres at first - and you should be getting at least 10 click/treats into those 10 metres - try not to click/treat absolutely regularly, so allow one slightly longer gap at some point, so Roxy has to keep concentrating on you. In the initial stages you will be click/treating virtually constantly - don't progress too quickly, as one of the main issues people have with clicker training loose lead walking is that the dog walks nicely, grabs the treat and then pulls ahead - if you are click/treating often enough there shouldn't be time.
Step 4. Gradually build up on this - so slightly longer between rewards in the same environment, introduce more distractions (with a high pace of reward), keep it going for longer.... don't build too quickly - and if you are taking it to a new environment, initially go right back to step one to build those foundations (you shouldn't need to repeat them as often). I think the major reason people struggle with loose lead walking (and I do - we use a T-Touch harness) is that they do the basics, and then jump to wanting perfect walking, missing out the middle steps of strengthening the behaviour.
If at any point the lead pulls tight, just stop and wait for Roxy to look at you - if she's utterly distracted, make some noise to attract her attention (but don't just keep repeating her name!).
Make sure you give constant feedback - especially when you are fading out the treats - tell her how clever she is constantly.
It's hard work - and can take months to perfect - but it's worth it!!!
While training is in progress, never ever let her pull on her collar and lead - if you are going somewhere and can't keep to the training, use a harness (or drive and let her off-lead immediately). The harness won't necessarily stop her pulling, but it will stop her learning that she can pull on her collar.
Also never pull her - the rule is that the lead must be in a "J" shape - it doesn't matter who does the pulling, this still breaks the rule!!!