Oof. That sounds like a very difficult situation for all. Based on what you've said, IMO, your preceptor is being too hard on you or at least not communicating with you effectively. Being a preceptor can be very stressful, and not everyone is cut out for the role. She seems a bit out of line with the comments about how that child would not have recovered etc. Also, some people just don't seem to understand that people who are learning need positive feedback as well as constructive criticism. You seem to be only getting the negative with no encouragement. You may want to take a deep breath and straight up ask her every day something like "I thought I did X well today and the mom seemed more comfortable with our plan of care; how do you think I did?" By asking her about something specific, but for which you've already noted the positive effect on the pt/family, you not only take responsibility for the pt interaction, you also are telling her that you can indeed critically think about pt situations, and that you are putting together your interventions and their results. It also should tell her that you are mature enough to seek criticism. Bonus: if her constant negative eval of you continues, you can then go to your manager and say that you have been seeking input from your preceptor on a regular basis; have shared pt/family feedback with her, but she doesn't seem to take that into consideration.
First time is the hardest. Frame the way I've suggested and unless this preceptor is someone who should not be a preceptor (it happens), things should improve, if only because you are taking control of the bit of this that you can. Hope this helps.