There is no reason why entertainers can't play roles that are geared towards children if they have roles that are geared towards adults. It is possble for them to divide these two parts of thier lives. This is particularly true when the role for children is as a voice actor, as many kids do not know who voices these roles.
This should be a non-issue. Free speech lets anyone voice any opinion through the mouth of any fictional character. If you don't like it, don't listen to it. Don't let your kids listen to it. It's as simple as that. I also don't like that rap artists are being targeted for this sort of censorship. I've heard many awful things come out of the mouths of nonfictional white entertainers/"journalists", and no one bats an eye.
I do not see any issue with Busta Rhymes being cast as the voice of Reptar. It would be a losing battle to keep children away from anything inappropriate in these days of prevalent and widely unmonitored internet access. In fact, any child with the initiative to look up the actor would be in danger of being exposed to far worse online.
Artists with mature lyrics should be allowed to voice children's characters. When an actor or personality voices a character their name and face is generally not used or known. Therefore, there is no tie-in with their former work and their current part. The children watching the show would have no idea of the artist or personality's former work and life. Thus, there would be no reason for children to find out about any past mature lyrics or inappropriate language. As long as the artist does not use that language in the children's show I see no issue with them being allowed to voice a part based on past music.