Welcome to Healthcare Magic forum.
Your daughter is telling the truth and is keenly observant.
Whenever we lie down prone, our heart comes closer to the anterior chest wall and has to exert more force against the pressure of surrounding organs on it .So this forceful beating can be felt as palpitations or felt or heard heart beats .
This is normal and you needn't be concerned.Same feeling can be experienced after running fast or during fever .
But if she persistently complains of palpitations even in upright positon or at rest then the pathological causes like cardiac or respiratory illness or anemia or thyroid hormonal
problems need to be ruled out .You should ask for associated symptoms like breathing difficulty, headaches
, fatiguability, dizziness
This can be done by a thorough physical examination and basic investigation by her pediatrician
So as of now this doesn't seem worrisome but do keep a watch for any associated symptoms.
Hope I have answered your query.