Is it ok for a child to still use a child's toothbrush even after she's lost most of her baby teeth?

Yes. There isnt much of a difference between a childs toothbrush and an adult toothbrush besides size. Many times the adult toothbrushes are too large (even from some adults) not letting you get behind and cleaning the last molars (most back teeth). The only problem with a brush being to small is not getting all the surfaces of the teeth enough so you might need to brush a little longer.
Yes. We prefer soft brushes, so that is fine, especially if your child's mouth is still small- a smaller brush will be more effective in cleaning the teeth.