Read Desirée Rumbaugh's reply:
It sounds like you have tried modifying Child's Pose to no avail for this student. I suggest you try a few other simple options for a resting pose that she can enjoy.
One idea is a forward bend in Sukhasana (Easy Pose), a seated, cross-legged position. This will work for most people—even those with very stiff hips, if they sit up on a blanket or two. Be sure that she holds her feet in a strong, flexed position so her ankles don't collapse, putting her knees at risk. If her head doesn't rest on the floor in this position, she can rest it on a block or a blanket.
Another simple option, depending on her flexibility, is Wide-Angle Seated Forward Bend (Upavistha Konasana), again with her head supported on a block or a blanket if necessary.
It will be empowering for this student when she connects with a resting pose that is right for her. The most important thing is that she feels comfortable in her body. That is the gift that yoga offers, and it is wonderful that you, as a teacher, wish to find ways to provide all of your students with that understanding and self-love.