Whew....a brush really is that magic tool for a painter....
If you're looking for a homeowner tool that your guys can make straight lines with, try this one. I tried one out once, and it works pretty good once you learn it, but I am WAY faster with a brush...not to mention just as straight.
Shur-line
Either way, even that tool is going to have a heck of a learning curve to make it work right......then you lose out on production. You can try it though and see what you think.
Any painter worth his weight in salt uses a brush though. I learned to paint with a 3" block....everything from doors, window sashes, trim, etc. If you can do it all with a 3" block, life becomes a cakewalk when you use a 3" sash in it's place.
I'd say take them boys aside and teach them how to paint straight lines. It's really a very simple thing to do when one understands how the brush works and is meant to be used, so teach them that and they will soon catch on. Make them cut in to baseboards and you handle the ceiling lines. If you're like me do one coat on trim, 2 on walls, then finish trim, there is plenty of room for practice along baseboard. Also, since you have a 2nd coat to go on baseboards, a little wiggly line won't hurt.....Find creative ways to train them and keep production moving at the same time.