I wish your question was more specific. How often does your child get a bloody nose for example.
All children get bloody noses from time to time. It's usually caused by the inside of the nose getting dry and the child scratching the buggers out or just scratching because it's uncomfortable. Or because they've bumped their nose. Sometimes they do when the weather is exceptionally hot and they've been playing rough. Sometimes for no reason that we know of.
If your child has a bloody nose often it may be because the blood vessels are too close to the surface. I had a bloody nose nearly everyday in the summer time. The doctor cauterized the vessels and the bloody noses stopped.
If your child has frequent bloody noses, I'd take him to the doctor. There are causes which require medical care. I don't know what they would be for a child. But my grandmother had a terrible bloody nose caused by high blood pressure. I seem to also remember my family saying that "thin" blood can cause bloody noses. I don't know what that means, tho.
Otherwise, if he gets a bloody nose every once in awhile I wouldn't be concerned. You might try lubricating inside his nose with vaseline. And depending on his age you might need to take care of buggers before he does.