I would put together a list of my accomplishments during the 90 day period (successfully completed the training program, assumed responsibility for the reading circle, introduced myself to the parents, etc) This tells her that you are taking this job seriously and will make her more likely to do the same. It shouldn't take you very long to do this and it's an excellent way for you to manage your career. You should do this for every review, or at least once/year. Then, when you leave the job and apply for another, why you've got your resume accomplishments all written!!

Then, I would email it (if you use email) and give her a hard copy. Say that you prepared this document in anticipation of your review and ask her if she would like to schedule a meeting time. If she puts you off, say "I understand you are busy (or, I understand you need time to review the document). Does it make sense to you if I come back on Friday during naptime (say a specific time and date) so we can schedule our meeting?"

This is called "managing your manager" and it's an important skill to learn. When you come back on Friday at naptime, start the conversation by saying "It's Friday at naptime and I'm back to remind about the review, as you asked!" Very important to say "as you asked," because it has the psychological effect of making her think the she asked you to come back and she will feel guilty about putting you off again. If she does, give her another specific time and day when you will come back, and then do the same thing again. Unless she's a total tool, she will not put you off very much longer.

Finally, when you actually do get your review and raise, say "Thank you so much! It was my understanding that I would get this raise 90 days after my start date, so I just want to make sure this raise will be retroactive to April 18th." If she says no, then say "When I selected this opportunity over the others I was considering, one of the biggest incentives was that you said I would be earning [$XX dollars per hour] after April 18th. I'm really hopeful that this commitment hasn't changed now?"

Here's the most important part: KEEP YOUR HEAD! Do NOT get mad, do NOT "lose your patience," do NOT get emotional. You are in the right and you have the facts on your side. If she does not keep her commitment, after you have clearly (and respectfully) reminded her of it, then you can assume that she will fail to keep future commitments to you. Then, you have a choice to make: do you stay there and cheerfully accept this or do you go elsewhere?

Hope this helps!