To me, this is trickier than it should be: you can have weight on you, invisible weight, and still look undernourished. There is no spell to make someone acquire fat and muscle.
I'm assuming this is eating disorder related, and so making her gain weight will only upset her more. Not to be harsh, but if she can't take care of herself, then how well does she take care of her children? She needs help and encouragement, not a spell. If it has gotten to the point where she may lose her children, then I think it is time for more concrete and direct help.
I'm so sorry to hear about this :( I wish her and her family the best!
The only magick I would advise is perhaps a ritual bath for her, and maybe an amulet or place on your altar (if you use one) to bless her and promote self-love.