Help docs! I'm trying to find out what causes irregular heartbeat in toddler while asleep?

Normal. Many children have sinus arrhythmia...Which is fancy for irregular heart beat. It is normal. As long as there are no changes in color...It is just skips a beat here and there...It is fine. If you are worried see your pediatrician. We see it all the time...No worries.( and if there any worries, then you will see a cardiologist.).