No, of course giving children chips occasionally is fine - we were talking about a diet of virtually nothing else!
All the evidence is that people who go on 'fad' diets (i.e. restricting certain food groups) not only put any weight lost back on, but gain even more.
We all know, don't we, that the only way to lose weight safely is by eating a little less and exercising a little more and not expecting any quick results.
There are, of course, some children (and some adults) who appear to flourish even though they have a very restricted diet. Our bodies are strange and wonderful things.