IMHO, blitz is good for kids. Most kids don't have enough exposure to chess. If your child only plays long games, s/he will hardly find any playing opportunities locally or online. The only places s/he can play might be tournaments on weekends. That will not be enough. For any opening, or any tactic skill, one has to see it, and use it often enough to master it. Playing games once a week is definitely not enough.
For the same amount of time, for example, one hour, you may only play one long game, including your searching time. But you can play at least six 5-minute blitz, which means you have 5 more chances to practice. Yes, it can't train your long calculation skill. You have to practice that in tactic training and master game analysis.
Most top players love playing blitz, including Carlsen, and Nakamura.