Need Lower Back stretches!

Hi everyone,

In doing FB abs, I'm working out my core like mad - certainly much more than I used to (it's the weakest part of my body, ha.) I've found my lower back is sore a lot more often and this can get uncomfortable during the day. I can never seem to stretch it right even with crazy amounts of child poses. Does anyone have any recommendations? And should I be worried about actually hurting my lower back?

Thank you!