Not telling painful words to one’s parents, or delivering to them painful news:
Included in the mitzvah to honor one’s parents in a speech, is to abstain from telling them words that will cause them pain. Likewise, one should not share with them news that will cause them pain. This may apply even regarding news that is relevant to the parent himself.
Telling them of a terminal illness that they have: Based on the above, one should not share with his elderly or sick parents that they have been diagnosed with a terminal illness if this information will not help them in any way and on the contrary will simply cause them great pain. However, if there is some benefit for the parent to be aware of their illness, such as to take greater care of their health, then they are to be told. Practically, one should consult with the medical team of his parent and a rabbi or chaplain prior to making a decision on this matter.
Telling one’s parents painful information about himself or his children: Based on the above, one should avoid telling his parents news and information of matters going on in his life that will cause them pain and stress, unless they are able to assist him, or need to know as part of their job of being a parental guardian.
 Betzel Hachochmah 2:55, 7-8