Well, all the clues are there already that Bella most likely already had a child. And due to some tragedy lost it or it died or whatever. In her bathroom downstairs you can see a step ladder and a mirror practically at the height to accommodate a child.
Also if you peek under the locked door and observe carefully you can see a pair of small shoes. Which many has speculated that's from the painting Bella removes from her small coffee room. So we can presume that this would be some reason for James to leave in the first place with having issue with dealing with the loss of his child, or James took the child away from her.
locked room peek
If the clothes Bella loaned the MC to do Yoga belonged to her son, then her son was already quite old, like 8-10 years old.