Gwen, dogs DO get jealous.

I would watch to be certain the youngest is not teasing the dog somehow, someway. Or unintentionally hurting the dog. Like if the dog sleeps in the child's room and the child fell out of bed onto the dog? Something like that.

And start them to build a better relationship. Do you permit any treats for the dog? Work out times when the youngest greets the dog and gives a treat, to reinforce the positive. Maybe when the older children are at school or out of the house so they don't want to be involved too. You want just the 3 of you - you are in the background really, just to make sure things go OK. The youngest needs to adapt to talking to the dog in soft tones (what I've heard some parents call "in door voice,") and pet the dog and give a treat. A SMALL nibble, even a piece of kibble, so maybe work up to 2 or 3 pieces over time. Child should offer the treat on the palm of the open hand so as not to be bitten. Is your dog grabby about treats? Then use a glob of peanut butter rather than a treat. You don't want to have to step in to correct the dog, if possible.

Hope that helps. My other suggestions is that you post this as a new thread to get more viewers.

Best
Sandra