I have a 12wk old flat coated retriever dog who seems to be challenging for dominance.  He bites my anckles and back of legs, shakes and growles, seems to be going beyond mouthing.  At the moment I say NO! turn away and ignore him unless it is too painful when I put him in his crate for a couple of mins. 
We eat first, ignore him for a couple of minutes when reuniting and follow the advice generally seen on the message board, with gmes toys etc.  He also becomes agressive when we take him off the settee, where we are not allowing him to go.  I have 2 boys 10 and 8 who are very good with him but unfortunately I often have to keep them away from the dog as I am worried he will bite them and this will put them off the puppy.  Before getting him I did loads of research into the breed as I wanted a gentle family dog fond of people and exercise. 

Can any-one advise whether this level of biting is normal at this age for this breed.  Previously as a child had golden retriever bitch with none of these problems. 

The grabbing of your ankles and shaking is puppy play. Keep doing what you are doing ignoring it, and use a toy to distract him from your ankles! The growling when you remove him is also something I have come across before. He is just being a young dog seeing how far he can go with you. I would suggest you contact the breeder of your puppy and ask for help, as they will be able to help you with your problem. All resoponsible breeders and flatcoat breeders in particular would expect you to ask for their help.
Another route would be to join flatcoats@yahoogroups.com which is a site dedicated to flatcoated retrievers and has many experienced and knowledgeable people there. You should also consider contacting the Flatcoated Retriever Society, as they have a network of area representatives across the UK, who are there to help people having problems with their dogs. The Society also has a list of trainers who are flatcoat knowledgeable who can help you.
Email for more advice on training,
regards
Polly & Foxpath Flatcoated Retrievers.

you state as a child had a goldern retriever bitch with none of these problem.

goldern are complated different from fcr,in temperment,fcr are the peter pan dogs that never grow up,and into every thing like a two year go you got to have eyes in the back of your head with a fcr,where goldern are more laid back than a fcr.

my fcr pup is 16 weeks and is doing the same play biting,but this is the age when they are lost their teeth.give him a kong and fill it up with some goods then but it in the frezer,and give it to him cold it will take a long time to chew and it be cool on the gums,
why are you keeping the boys away give from the puppy,the boys should to be told how to control the pup to with words,and not to do rug of war games with the pup,it will not  do you and the boys any good by keeping the dog away from them it will make it worse,beacuse he knows that  he has won over the boys,you just got to be firm but kind with this breed to heavy handed with this breed will turn the dog inwardley and a nervous wrack,

have you spoke to the breeder of the pup about the sitution,have you gone to puppy classes as a family,if it getting to much you must ring the breeder,first,

I agree with Briedog a kong toy filled with treat and put in the freezer is a good play thing and is cool on the puppies gums, you can buy these at most pet stores, or from the Company of Animals, via their web site.
The growling is just one sleepy puppy saying leave me alone please. He is saying this but you are doing the right thing and removing him, he will give up growling if you are persistent in doing this. It is not a dominance or aggression problem. He is testing his boudaries just the same as your children will push you to test their boundaries.
Play biting is something that dogs grow out of, but you must do your part and offer alternatives. What do you think attracts him to your ankles? Is it because you wear shoes with laces for example? Do remember a dogs eye sight is geared to be attracted by movement too. Owning a flatcoat is an exercise in thinking ahead, try to see things from your pups point of view. Raising a wellbehaved dog is very mush the same thing as raising a well behaved child, both push boundaries.
As Briedog says separating the pup from the boys is sending the pup the message he is higher in the pack than your boys. Firm kind control is required and the children should be made to understand they must leave him alone when he is tired and sleeping, and be taught how to help by using the training methods you are using. The children will also need to be very aware flatcoats will pick up and carry off, (to chew). their toys, so all toys should be put away safely while not being played with.
If you could tell us where you live we could always help by advising you of a good trainer near you. Goldens, labradors, flatcoats, curlycoats, chesapeake bay and duck tollers are all retrievers but each breed is different in temperament and how they react living in the home. A golden will happily sleep and fit in, a flatcoat needs more attention and will seek it in whatever way it can. Teach your flatcoat little things it can do for you, and he will become the most loyal and trustworthy companion you could wish for. The reason the flatcoat needs a lot more attention and stimulation is that the breed is still very much a working breed.

Just to say that the above advice by Polly and Briedog is very sound.

When we have pups, a friend does a character assessment test at 7 weeks old.   This will pinpoint the stronger-minded pups in the litter (I hesitate to say dominant with a flatcoat - I have never had one who is dominant with people).    We then suggest the most suitable pups to the new owners, and I don't think we've got it wrong yet.   Mind you, we spend so much time watching pups that we've got a pretty good idea of characters before the test is done.

The new owners, I suppose, are then restricted in their choice of pups, but it saves the pups being placed in an unsuitable home.    New owners also rather like getting their pup's '11+' test results on paper.

Jo and the Casblaidd Flatcoats

like you say jo,us breeders watch the puppys characters over the 8 weeks while they are with their mum,again i do not let the owners choses which pup they want,i will guided them to hopefully the best pup for their sitaution,
with 3  litters i have breed, i dont thing i got it wrong either,again the breeder know best,not the new owner.we can only guilded them.

http://www.jersey.net/~mountaindog/berner1/bitestop.htm