their knees
Mi padre me tenía sobre sus rodillas mientras me contaba canciones de su tierra.
My father held me on his lap while he sang me songs from his homeland.
Tom sentía temblar sus rodillas.
Tom felt his knees trembling.
Su coche estaba estancado en nieve que alcanzó hasta sus rodillas.
His car was stuck in snow that reached up to his knees.
Ella sentía temblar sus rodillas.
She felt her knees trembling.