That depends entirely on why your child hates your boyfriend.
Keep in mind that you bore a child, and now bear a responsibility to care for that child until s/he has reached adulthood. The needs of your child should always come first.
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You don't have to like your mom's boyfriend. It is your mom's prerogative to have boyfriend she likes (whether or not you do). It is best for you to understand why your mom likes him rather than focus on why you hate him (your reasons really don't matter)
However if you hate him for a specific reason, you might tell your mom the reason why you hate him (when he is not around, of course!). This is of not much use either unless you feel compelled to do so.