I feel that it would make sense to put them all in one bucket, so I agree with the proposed suggestion: make baby/child-proofing synonyms of safety.
We currently have 64 questions tagged safety. If we introduce baby/child-proofing as tags in their own right, then some of those existing questions need to be re-tagged. I see these options:
0 new tags: *-proofing are synonyms of safety.
Pro: no re-tagging needed, amount of content remains manageable.
Con: I can't think of any drawback. I don't see it as a problem that safety and *-proofing aren't separated.
1 new tag: child-proofing, with baby-proofing as a synonym.
Pro: ?
Con: Need to re-tag existing questions. Room for ambiguity on when to use this tag vs. safety, or when to use both.
2 new tags: child-proofing and baby-proofing are separate tags.
Pro: fine-grained.
Con: Too fine-grained; I don't see the need to separate them. Also, this would leave room for ambiguity on which one of the tags to use, and also when to add safety.