A friend of mine dealt with this issue with her son at about 3.5 years. They removed his tonsils and adenoids, and his life turned around. He was a whiny, shy little boy who wanted only his mother. When he started getting enough oxygen while he slept he not only slept better, but he became a happy child who now can't be stopped.
Sleep apnea is VERY serious. I have it myself (and have the Darth Vader mask LOL). If removal of her tonsils will help or eliminate it, then I advise you to proceed. Be sure to see a reputable ENT before making any decisions. All surgery carried risk, as you know, but if your child does not get enough oxygen to her body each and every day, the cumulative effects can be devastating. Sleep apnea is exceedingly stressful on all organs, including the heart, and of course it is bad for the brain.
My best to you.