Why would a normal child stop breathing for a few seconds while he's sleeping?

Yes. Transient pauses of up to 10-15 secs are often seen in infants. After that time brief (<10 secs) pauses are seen, longer pauses are worrisome.Frequency & depth of breath ore controlled by brain centers & sleep requires less rate/depth than wakeful periods.Any kid who falls asleep rapidly during the day or naps while his peer group is playing may have sleep apnea & deserves a workup.