I wouldn't put her in underwear until she's fully trained. No go there. Drop it for now. When she is ready, she'll let you know. Be sure you get a potty chair on the floor that can be hers.
Many children are afraid of sliding through the seat of a regular toilet.
The incidence of failure in training is rising again. Some other experts and I recently had a conference in Chicago at which we discussed this recent rise. We think it must reflect pressure that children are feeling. Parents should take care of toilet training their own children. All of these amount to pressures that not all children can handle.