Does "makaslar" translate to "pairs of scissors"?
makaslar is plural here. if you are suggesting a use like "a pair of pants", no. in turkish one pair of scissors are makas, or a pair of pants is always singular "pantolon". in this sentence, it translates to more than one tool.
Why not seninki?
senin sounds more natural to me. actually that sentence is 'kırmızı makaslar senin makasların mı?' but shortly we dont say makasların and we just say kırmızı makaslar senin mi?
The correct translation should use "makas", instead of "makaslar" since in turkish the later refers to more than one pair of scissors.
And in English "scissors" can refer to more than one pair of scissors as well.
Imagine that there are five pairs of red scissors and three pairs of blue scissors on the table. "Are the red scissors yours? Kırmızı makaslar senin mi?"