I think it would benefit your son to learn to tolerate the toy. I am sure that there are other ways for your 4-year-old to learn her letters, but this is also a learning opportunity for your son. If you decide to bring out the toy, try to do so in a sensitive way that will help your son learn to tolerate it. At first, allow him to be far enough away from the toy so that he can't hear it, although he knows it is around (this will also require cooperation from your daughter). Work with him to identify an appropriate way to handle the situation (e.g., a relaxation technique, staying busy with something he enjoys). When he does well, gradually expose him more and more to the toy. If this helps, you can apply this technique to other sitations that are problematic. If at any point you encounter a severe reaction (e..g., aggression, destroying property, self-injury), please stop and seek the assistance of a Board Certified Behavior Analyst. A list of professionals is available at the website of the BACB.