at your mother's knee字面意思是在妈妈的膝盖上，我们小时候就经常被妈妈抱在怀里，坐在妈妈的膝盖上，所以at your mother's knee意思是年幼时、小时候。

【例句】
I learned how to bake at my mother's knee when I was just a girl.
小时候，只是个小女孩时，我就开始学习如何烘焙。

I learned to speak French at my mother's knee.
我孩提时就学了法语。