The ace or f2 would probably suit better as backup or loaner machines but the youngsters tend to get confused with multiple tones and buttons and vdi numbers. The compadre is just so simple.... but I have heard some complaints about the 8 inch coil model. But the 5 inch is lighter and easier to pinpoint for kids anyway... also, don't forget that lifetime warranty!
I only have experience on a Garrett and it is a ACE 350 which I like. The 250 is very similar and for coin hunting would be great from what I read. To keep them interested I would plant a few coins discretely and watch their eyes light up when they discover them. This will more than likely hook them into a great hobby. I even get discouraged when swinging all day and no hits. But I keep the right frame of mind on it. Not every day will be a success. If I find about $1 in change in a hunt I am happy. Still looking for that valuable coin or at least a old coin that has real silver!!!!
Good luck and as far as the other brands mentioned, I am sure they are just as good. I do like the ACE readout, easy to understand and usually spot on in what you find.