Last Saturday our 16 month old GSD started to act strangely towards the kids.  By this I mean if they approached him or our younger dog( who is a bitch) to smooth them he growled.  It has started again today.  He doesn't bat an eyelid if they are playing with our 3 year old gsd. I can't understand what is wrong with him.  We are booked in at the vets in the morning for a check up, but I am convinced it is one of two things, either he has suddenly developed a fear of the children, as when he growls he starts to shake or the dominance side is kicking in.  Bit worried as he has been brought up with the kids and has always had a good relationship with them.  He is good with other dogs, cats and is a joy to own temperament wise, so I am very worried with what is going on now.  Any help appreciated.

Not dominance, he is resource guarding; his bitch!  He is at that age. :)  Difficult to advise over the net as one really needs to understand the dynamics in the house which may be a bit complex with 3 dogs, children plus adults.  You would probably find it more effective to have a 1:1 with a professional behaviourist who will talk to you at length and advise the best course of action in your particular individual circumstances.  Your vet will be able to refer you.

Good luck