Yes swallowing a marble can be dangerous especially if a kid swallows a marble.
Marbles can easily choke a child and especially if the child is a toddler or younger child so you need to keep marbles and other small stuff like that out of a child's reach.
If your child has swallowed a marble and is breathing then they will likely be okay but if they begin to choke you need to get the marble out by doing the heimlich on the child or toddler to hopefully get the marble out.
If you cannot get the marble out and the child is choking call 911 but still try getting the marble out.
Don't let kids swallow things they shouldn't.
If you notice they are putting something such as a marble or rock in their mouth get it away from them or remove it from the child's mouth right away.