"Dad's stomach is so warm" "Idiot, I didn't eat you, you're just in my mouth." *Cue symbolism*
Heart's Relationship with Pero-Pero and how he met her.
Hearts Mom taking care of Heart in general, even though she knows what may happen.
Baku realizing Heart is his son in the midst of their fight and allowing everyone to walk away after knocking Heart out.
The latter part of their fight actually somewhat resembles Heart's training of Umasou. Viewed from this angle, Baku is taking the opportunity to teach his son in the way that he was never able to.
Heart going on a rampage looking for Umasou while he's getting berries so he and his dad can eat together.
Heart meeting his siblings.
Early in the film, Baku asks Heart if he's hungry, and if he's an orphan. He's basically offering food to what he thinks is a lost little boy, then lets Heart leave and tells the other Big Jaws not to bother him.
Tiny little Umasou, when Baku is standing over the unconscious Heart and his family, hops up on Heart's chest and waves his tail back and forth in an effort to protect his daddy!