Hi, we took out 3yr DD to a sleep specialist. Best thing we did - we put up with 2 loooong years of broken sleep - not good for DD or US!! It put a LOT of strain on our relationship because we were continually grumpy & in zombie mode. Please get help for the sleep issues asap - don't leave them for so long like us !
The sleep specialist booked us in for an overnight sleep study - they hooked DD up to all sorts of sensors - electrodes, Co2 etc - then DD slept in hospital while hooked up & they took the readings. The wonderful nurses made an adventure out of it and she watched her DVDs, then she slept in the hospital bed while I slept in a cot next to her - she was not upest at all (we still have her photos of it on the fridge!)
It turned out that DD's problem is that her slepe waves aren't "flowy" in a normal wave pattern - they are quite jumpy in a staggered way - she doesn't transition well between light sleep and deep sleep. To counter this DD Is prescribed a tiny dose of clomazepan (anti-seizure drug) that has a nifty side effect of helping sleep issues. THe dose is tiny, and we wean her off the drug every 6months to see how she goes naturally - sometimes it resets the sleep patterns, so far not for us though! The Dr said that some kids eventually grow out of it at approx 6-7 yrs.
Highly recommend getting assessed by a proper sleep specialist - there may be a medical reason for the sleep issues. Hope this helps !