Well, it depends who made the chicken nuggets. If they’re from a reputable supplier, then yes the filling is basically made of chicken. However, it may be what they call reconstituted meat, which means that they take a mixture of pieces of meat – a mix of muscle, fat and other tissue – mash it up, and shape it into nuggets. Personally, I don’t have a problem with that as an occasional snack, but I try not to eat too much heavily processed food, so I wouldn’t be eating chicken nuggets every day. The coating is made of other ingredients obviously.
I believe there are regulations (at least in Europe) that force anyone making chicken nuggets to ensure they are actually made from chicken.
They may not use the best parts of the chicken to make their nuggets, but it needs to be chicken.