If a child isn't making normal sounds / not breathing then 911 should be dialled immediately (000 if you're in australia).
If your child is breathing but seems to be coughing / wheezing then the object is probably lodged somewhere in the windpipe. Generally the child is not in immediate danger, but ensure you take your child to the nearest emergency room as soon as possible.
If your child appears normal and is not having any of the above problems then it is not life threatening but it is still advisable to contact a paediatrician or your doctor.