I have a bad smell coming from my kids room only. The smell is only coming from this one vent. The smell is constant whether the heat or a/c is on. Please help

If your child is old enough to walk, could he have dropped some food or milk in there to spoil? Or, a mouse may have died in there. They can squeeze through really tight spaces. Do you have a pet that could have urinated in it? I've heard of cats doing that.

You don't describe what the bad odor is like (chemical, rotting, burning, nauseating, etc.) so these are possibilities that come to mind.