I don't know if I'd label child-hood obesity abuse, but it certainly seems like neglect. They're neglecting their child's health for convenience, eating convenience foods instead cooking anything. They neglect to teach their children constructive ways to deal with emotions, instead turning to overeating. They teach overindulgence instead of moderation. It's very sad that it seems generational, the parent was neglected in the same way and now is teaching their child an unhealthy lifestyle.
Idk, with obesity, there are very likely effects such as high blood pressure, sleep apnea, much higher risk of diabetes, higher links to certain cancers, increased risk of heart attack (due to higher cholesterol) and stroke. While the cancers, heart attacks and strokes generally do not happen in children, a very high percentage of obese children grow up to be obese adults (70%). Because of these negative health aspects, obese people tend to have a lifespan that is 6-7 years shorter than non-obese people. To allow something to happen to your child's body which absolutely will cause some of these side effects, including early death, could be labeled as physical abuse.
Then there is the emotional side: obese kids face a lot of social discrimination from their peers (being called the fat kid, not being picked for recess games, etc), which can, and probably does, lead to lower self esteem. Not being able to keep up in sports or be winded after a very short run is probably humiliating, which adds to the lower self-worth. Being teased because of physical limitations (not keeping up, etc) can make a kid feel isolated from his peers, lonely - feel not accepted by other kids.
If a child is suffering these psychological effects due to the physical effects caused by obesity, then I would venture to call that emotional abuse.
I'm pretty sure the parents aren't saying, "I'm going to abuse my kid by letting them get grossly fat, mwahahahaha", but yeah, it is a neglectful circumstance, like you said. Just one that has dire consequences.