I think you have to look at individual situations but given that some obese children have the nutritional intake of a starving child, then yes, I would consider that abuse (neglect if I had to categorise); you wouldn't stand by and let a child starve so why is this any different.
Re: thd individual situations, some parents are just feckless but once it's pointed out to them and they're given a plan then therd's a big improvement: most plans are not for kids to diet but rather to maintain a weight until ghe age that the weight and height is appropriate. However some families do not engage, they enable the child and look for medical causes where none exist: I think these children should ag the very least be on a child in need plan if not a child protection plan. I'm talking 11 year olds weighing 15 stone anc adolescents weighing somewhere in the 20s.
One thing that should be noted; when a child gets very big then depending on family dynamics, they can become very intimidating to parents who are trying to put measures in place to help them. I would argue a child protection plan is needed here too in order to help the family manage as on top of the food issue, ylu zlso have an isdue wherd the family dynamics are very twisted tobsay the least and help would be needed to untangle that as well.