Email salutations for mother / father Wikipedia lists many honorifics for one's own family, but I don't see any for one's own (living) mother or father. How would an adult write the equivalent of "Dear Mom" or "Dear Dad" in an email? Does "亲爱的妈妈" sound childish?
You are always a child in mother's eyes,So ... childish is acceptable .

You can also use it to your close friend :

亲爱的苏：

亲爱的阳：