Puppy aggression?

we have a 12 week old male mastiff x rottie. we have had him since he was 8 weeks and got him from a rescue home. he is a lovely pup, slowly picking up the house training, will now sit for treats. the problem we have is what we think is aggression towards our children. obviously he is always trying to eat peoples feet and we have taught him 'leave it'. he will listen to this almost 100% with myself and my hubby but never with the children we always have to step in. if i go upsatirs for anything, usually a shower or even to put away some washing he is unrelenting with the kids to the point of hurting them. he constantly snaps at their feet and jumps at them mouthing. i now just leave my 16 year old with the pup and put the younger 2 ( 12 & 10) in another room while im upstairs. but he is a nightmare! yesterday my 16yr old said 'mum its a different growl now he doesnt like me'! as soon as i come back down and tell him he stops! i started yesterday geting my 16 yr old to prepare and give him his food thinking this might make him see that she is 'above' him in ranking and it worked ok. but no sooner than im out of sight he is off again! does anyone have any answers or tips?
Thanks

This would be better in the other animals section, but even though this is a cat forum you can get some tips here, too.

Simply put, you need to make it clear to the puppy that he is to listen to all human members of the house hold. Puppy needs to learn this about any other animal members of the house hold, too, if you have any. The children must be firm, consistent to handling/training you use, and never show any fear.

Others should be able to recommend the proper books for you to pick up some wonderful tips from. But what you're facing now is a direct need to training. You could also look up dog trainers/behaviorists in your area and see if they are willing to work with you one on one if you think you'd learn better that way. You can't put it off, you're dealing with a dog that will get big and powerful.

Thanks
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Puppies for sale

All puppies are mouthy. They are babies. Always supervise your kids with your puppy and any dog really. Young children can't always be counted on to have manners and not hurt the dog puppy. Puppy needs to learn manners. Your puppy sounds bored. What does a normal day look like for your puppy?
Puppies need plenty of free exercise, some short walks, and lots of great toys to keep them busy while they are full of wonderful new life energy and teething. Does your puppy take any training classes? Most puppies benefit from training classes to exercise their mind, helps wear them out and teach you how to deal with behaviors you find less than desirable. Also you will have access to a trainer to help you in person to see the dynamic between you and your brand new pet.
Here is a link to find one close to you:http://www.akc.org/events/obedience/training_clubs/

Does your dog get enough exercise too? If you think all is well from feeding to training, maybe consider an obedience training for him..