Your original sentences are mostly clear, but here is a slightly improved version for better grammar and flow:
Original: Magpies are very smart. They can see themselves in a mirror and know it's them, not another bird. Magpies also make rattle-like sounds.
Corrected: Magpies are very smart. They can see themselves in a mirror and know it is them, not another bird. Magpies also make rattle-like sounds.
Explanation: 'It's' is a contraction of 'it is' or 'it has.' For clear writing, especially for young learners, it is better to write 'it is' instead of 'it's' to avoid confusion. The rest of the sentences are grammatically correct.