Is it just laziness or not thinking it's a big deal, like how my kids won't take coats to school on cold days?
Or is it something more? This would be a small minority of cases, but some kids don't shower to repel others, because of a trauma or anxiety. They intentionally want to be unattractive. Obviously that would warrant professional help.
My daughters looooove showering (we have endless hot water, so they take way too long). One of them showers every day, and one of them is more grungy and goes every 2 or 3 days. If she smelled, I would say something, but she doesn't. I don't personally believe you need to shower every day (and especially don't need to wash your hair every day) if you weren't perspiring a lot and don't smell bad. But that *if* is important.