Reddit reviews Finding the Right Spot: When Kids Can't Live With Their Parents

Here are a couple websites to check out:

http://www.heretohelp.bc.ca/visions/parenting-vol2/talking-to-children-and-youth

https://www.pa-fsa.org/Portals/0/Docs/Parents/Challenging-Situations/Helping_Children_Understand_Mental_Illness.pdf

You might also try reading some age-appropriate books with her that address those sorts of issues. Here are a few possibilities, some may not be relevant to your specific situation, or you may need to rephrase things a bit while reading, but hopefully it's a start...

Finding the Right Spot: When Kids Can't Live with Their Parents

Kids Need to Be Safe and Families Change - note that both mention foster care

Our Gracie Aunt - kids are placed with their aunt because their mother is unable to care for them

Let My Colors Out and The Invisible String - coping with situations involving parental illness or separation

When Mommy is Sick - mom is hospitalized, for reasons not specified, though she does come home at the end

[When Someone Has a Very Serious Illness] (http://www.amazon.com/When-Someone-Very-Serious-Illness/dp/0962050245/) - workbook format

[Wishing Wellness: A Workbook for Children of Parents with Mental Illness] (http://www.amazon.com/Wishing-Wellness-Workbook-Children-Parents/dp/1591473136/) - intended for older kids, but might give you some ideas