1. The violin is more difficult than the piano.
2. A violin is more difficult than a piano.
(Does #1 mean #2? Can we use #2?)
3. A child needs plenty of love.
4. The child needs plenty of love.
5. Children need plenty of love.
(Are they all the same in meaning, in generic use?)
6. Do you like horses?
7. Do you like the horse?
8. Do you like a horse?
(Are the three all grammatical and in generic use?)
1 and 2 are close, but not identical. I think you should use either "the" or "a" with both nouns.
3 and 5 mean about the same thing. 4 seems to be referring to a particular child.
6, 7, and 8 all are fine, but they don't necessarily mean the same thing. Do you see why?