“Ira” necessitates that some movement will happen (in other words, it’s a verb of movement in the future tense), while “va” doesn’t.

Take a sentence like “Elle ira travailler chez elle demain”. It only makes sense if she’s not currently at home. The same sentence with “va” wouldn’t have the same truth conditionals, it’s fine to use whether she’s already home or not. In order to have an equivalent sentence using “va”, youd need to use “Elle va aller travailler chez elle demain”

There is no great difference since the respective tenses are two future tenses, the “futur immédiat” and the “future simple”, but that is true only as far as the action per se is concerned.

There can be a subtle difference; in the first case you are dealing with a matter of fact statement. In the second you can stress « demain » and then you are saying that he is not going to do it now or at another time, you are saying that some other time than tomorrow that has been considered will not do.

Here is another subtle difference; the “future simple” can be used for a sarcastic effect. The pronunciation, of course, is crucial in the conveying of the sarcasm and I will not go into that as the discussion is beyond my means, but let it be said that the “futur immédiat” is not proper for this purpose. The sarcasm is to the effect that such a thing is a trivial occupation in comparison to something else that is in question.

• — Pourquoi ne pas le laisser lire ses Mickey Mouse et ses Donald Duck ?
  — Quand son instituteur l’interrogera sur ses leçon il lui récitera Mickey Mouse !
  “il va lui réciter” is not proper, but it must be said again, the intonation is very important

As the first form is used for matter of fact statements it is used much more than the second (ngram).

An important difference lies in what prompted someone answering with either one of those forms : if the reply is meant to tell someone that the boy/man will not be able to do something else in reason of his playing with a kite, you must use the form in the “futur immédiat”; the other one does not convey the message idiomatically.