Hi L.,
I have 2 boys, 4 and 5 1/2 and I started cutting their hair at home after my first sons visit to the beautician when he was a year old. I too found it ridiculous that I was paying $20 for her to cut his hair when it looked terrible because of all his wiggling. I figured I could make it look terrible and keep my $20 each time :) You're boys are older so they may be able to sit still but I can put them in front of a movie and it keeps their attention while I cut their hair. I do use scissors and it took me a few tries before I figured it out but for me it was just trial and error. My boys were younger too so they didn't care what it looked like.
On the top I comb their hair straight forward to their forehead, I form a mohawk between my fingers and start there and cut the top part long ways. As I get to the sides, I cut up and down and I cut the sides shorter (the width of my finger as I pinch their hair between my fingers to cut). As I transition from the top of their head to the sides I taper it a little to make it blend together. I do have clippers that I use around their ears and on the back of their neck.
I hope you decide to to it, it's definitely worth the time and money! If you do it, for the first time, cut long. Whatever length you think you should cut, give it an extra 1/3-1/2 inch. You can always cut more off but you can't put it back on! :) You can also sit them down before they really need a haircut and just take a tiny bit off, just to get the feel for it and a little experience without a dramatic change. The clippers didn't work for us, buzz cuts looked terrible on my boys. Good luck!!