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Assuming your son's telescope has focal length of 1200mm and focal ratio of f/8 and you have a 1.25" focuser, I'd suggest you add to your eyepieces rather than replace.
Your 10mm eyepiece gives 1200/10= 120X (Exit Pupil 10/8= 1.25mm)
10mm with Barlow= 240X
Your 25mm eyepiece gives 1200/25 =48X (Exit Pupil 25/8= 3.125)
25mm with Barlow= 96X
If you want the widest field of view consider getting a 32mm eyepiece which gives 37.5X . If you get a 40mm eyepiece in 1.25" barrel size, it has a narrower field, and you see as much as with a 32mm eyepiece. But if you had a 2" focuser, you'd really want to think about a 2" 40mm eyepiece.
A 32mm eyepiece has a 4mm exit pupil, which makes the image look brighter (including background skyglow), and a 40mm eyepiece has a 5mm exit pupil.
Highest power for an average 6" reflector - which is well collimated, and on a perfect night (which don't come around that often) is around 50X per inch, or 300X. In your telescope, that's a 4mm eyepiece or 8mm eyepiece with Barlow. Unless you buy a more expensive 4mm, like a TMB one, the Plossl ones work - but their eye relief is very tiny. I had to take the rubber eye shade off mine to be able to use it. Not that pleasant to use.
The 6mm eyepiece you were thinking of getting gives 200X, which should be useable on very many nights (providing your son doesn't aim over houses with lots of heat coming off them).
If your son one day really gets into it, goes to dark sites, hunts galaxies and other faint objects, he'd want a 2mm exit pupil eyepiece, which equates to a 16mm eyepiece. But I think that will be a long time off.
To sum up, my thoughts are that after getting the Barlow, you should be pretty well right for high power, but that you may want to consider getting a lower power 32mm eyepiece.
Regards,
Renato
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