I don't practice law in WI but generally you cannot just "take custody." But if she is willing to let you have your children that would likely be ok.
If she is is not willing to let you have the children so they don't have to go to the shelter with her, it would seem that her need to go to a shelter would likely qualify as a reason to let you advance your review and get in front of the court sooner.
Talk to the GAL about this new development.
File a motion in court to change custody and placement. You need to allege a substantial change in circumstances, which clearly her moving into a shelter is such a change, the court then can order mediation, a guardian ad litem or authorize an immediate change.
Sign up to receive a 3-part series of useful information and advice about child custody law.