At what age must a mother cover her 'awrah from her child? And is it permissible for her to wear short clothing in front of her husband and child together?

If a child can discern 'awrah (intimate parts), it is not permissible for a woman to uncover her 'awrah in front of him. However, if he cannot discern due to his young age (such as a child aged 11 months), then it is permissible. The determining factor is the child's discernment and his understanding of these matters. If he is four or five years old, he might be able to discern, and in that case, it is not permissible to uncover the grave 'awrah in front of him.