You can not use "en" in here. Like Guillem91 said, barn is a neuter noun and therefore takes "et" as indefinite article. It is like in English, where you do not say "an child".
However Danish people will still understand what you are saying, if you use the wrong article :)
My guess is that it is a neutre noun, so et must be used instead (like Kind, in German, which is neutre, or when in English we people use "it" when talking to a child). So the word child is likely to be neutre in at least most Germanic languages...
But it's just a guess!
Can it be 'barnet' i stead of 'et barn' Whats the difference I think ive already asked this question before
i heard that we use ''en'' for the things have gender and ''et'' for the things doesn't have gender. then why we use ''et'' instead of ''en'' for it?
Grammatical gender, not the actual gender of the thing. Just like in Portuguese clocks are male and chairs are female (we don't have a neuter gender), in Danish words are either male/female (en) or neuter (et), and it's completely arbitrary.