Apply a straight solution of Murphy's Oil Soap directly to the stain and allow it to penetrate completely. After soaking for several minutes, brush the stain with the tooth brush.
If any stain remains, apply a small amount of Ivory soap to the stain and work this in with the tooth brush.
Another option is to work at the stain with a typical baby wipe. Use the baby wipe in a scrubbing fashion and many times this will remove a dry erase stain.
Often a Mr. Clean Magic Eraser can be used to lift out dry erase marker stains. Attempt to rub the stain out first with a dry magic eraser. If this isn't effective add a small amount of water to the magic eraser and rub at the stain again. This usually works well.
Wash the garment as normal in the washing machine.
Check to make sure that the stain is out, prior to putting the garment in the dryer. Once the garment is dried, the stain is set and cannot be removed.
There is also a stain removal product specifically for markers on fabric: Motsenbocker's LIFT OFF. Home Depot carries it. I have not used that particular stain remover, but their other products work well. FWIW, I had almost exactly the same problem a number of years ago: green marker on a quilt. Wish I'd had all the advice from Dar back then, or had known about the LIFT OFF.