Urban Angels are a good company to go with, they are quite tough with applicants but are a know good company, although i think the London branch is probably better than the newer Northern one. Also try Bruce & Brown, which is extremely highly thought of too.
The *best* agent i ever used was Stagecoach, i don't know if your little ones attend a stagecoach theatre school, but if they do i'd almost certainly sign up for their agency. The best thing about them is they don't solely rely on looks alone; if you think your boys are little thesps, they allow them to grow and take LAMDA certificates throughout their time with SC.
The downside is, it is expensive taking them there, but it could be arguing that child modelling costs a lot too (you'll truly struggle to get an agent who doesn't charge, whatever the rights and wrongs are). The upside is, my son whose now 15, has a raft of LAMDA qualifications he can put towards his CV and also has been lucky enough to work on several occasions for the Beeb and loads of clothes companies too.
Good Luck and remember at the end of the day, just because your with an agency it doesn't guarantee work, mostly if you go for a casting and your child doesn't get the job, it just means they didn't entirely fit the casting directors brief.
Its good fun and whatever any says, if the kids enjoy themselves, they do get a small savings fund for when they're older and it does encourage them to be outgoing too.
:)