How old is Tristan? Is he old enough to give Scout his food? Or, to tell Scout to sit and stay before he gets his food? Somehow Scout needs to learn that Tristan is higher up in the chain of command than he is. What Scout is doing is try to assert himself to be higher up in the "pack" than Tristan is. Nipping in a pack of dogs is not going to cause injury to another dog, but it of course will to a toddler. Scout doesn't understand this. To him, Tristan is another pack member and you and your husband are the pack leaders.
So, what you need to do is train Tristan while you train Scout, or rather, have Tristan assist in the training of Scout - even if it's just having Tristan by your side as you train/discipline Scout. If Tristan is old enough to give Scout his food, then Tristan will start looking to Tristan for what he needs: food, attention, direction, etc. and should revert to a more submissive position in the "pack".
If Scout hasn't taken an obedience class yet, take him but have Tristan go with you.
Does this make sense?