You are not going to discourage your dog from loving children, or even wanting to show them affection by training him to act in a responsible way. You will also teach him that the new way is more enjoyable (this is the key to actually enforcing the new behavior). Try to be with your children when you start the training exercises and brief them in advance on the appropriate behavior for them to demonstrate. Your dog needs to learn that he will only be petted by your children when he is sat down. Much like teaching him to sit to receive a treat, he will learn that unless he is sat he gets no attention.

As soon as your dog starts showing "too much" affection, your children should turn their backs on your dog and become silent and stationary. As your dog rotates to see their faces, they should continue to turn. I am assuming at this stage that your dog does not need to jump on them to knock them over. The importance of you being there is that you need to instruct him to sit. In the beginning, give him a small treat and tell your children to turn and give him affection. Every time he stands back up, have your children turn their backs and be silent again.

This will take time, how long exactly depends on your dog (and your children’s discipline), but you are likely to see rapid improvements in the short-term, even if not every time. Remember that he will get over excited from time to time.