Hello!
Maybe first of all .... how old your sons are or what you allow them to do is your thing - I think most of us played games before we were allowed to play them ;)
I've seen 15 year-olds with 1,5 years of BF2 experience play better than 40 year old men with 3+ years BF2 experience, some are just good at the game.
If you want to check if your sons are cheating, it's best you stand behind them while they are playing and look at their screens. Most cheats have some kind of overlay or alter parts of the game (e.g. you can see an aritificial crosshair, vehicles show up in funky colors, you can see where everybody is on the map, ....)
As to prove it to the clan - this might get tricky. They can take pb screenies of you and your sons (I hope they don't come back black) and check you out on PBBans, watch you play ingame (spectate you) and if they have battlerecorder use the demos to observe you further.
But - and this is a big BUT - if you get accused of cheating in a clan I'd definitely reconsider hanging around there for much longer. It certainly makes a difference whether that person accusing you or your sons is the leader or another clan member. In the end though you'll always be their suspect and until they're better than you they will keep accusing you of cheating. That's the typical case of "I can't lose" and there's nothing you can do about it.
I think the staff will help you further :)