In short, yes. Of course, it’s vital that you know what you’re doing, follow instructions and launch when you have clear weather and an open space so that you take every precaution to avoid any incident, as some materials used can be flammable. But, model rockets have been launched as a hobby for decades and this wouldn’t be possible if they weren’t safe.
If you’re introducing younger kids to the hobby, you can use rockets that have propellers or are water-powered and don’t use flammable materials for an extra safety measure.