Bob's suggestion will allow Mary to find a middle ground between her family and her career.
Explanation:
Mary will be able to follow her career path which is rewarding and lucrative if she does not have to work full time because if she follows the path she will have to work part time and she will still have time to spare for her children.
The same goes for her family life because if she had to have chosen the job she would not be able to take care of her children and give them the upbringing she wants to, if she had left the job she would have to let go of her career.