The content of your poem for your baby niece will depend on a number of factors, such the occasion for which the poem is being written and your message to your baby niece. You can tell a story in verse format with your baby niece as the central character, or just describe your impression of her and convey your blessings and good wishes.
If your niece is newborn, you can write her a poem welcoming her into the family. You can put in little details introducing each family member to her. Alternatively, you can even compose a poem expressing the happiness her arrival has brought to the family. If your niece is already a few months old, you can tell her a little story where she is the heroine, through which she can learn an important life lesson when she is old enough to read and understand the poem. Your poem can also be a ghost-written message from her favorite toy or cartoon character - you just need to know what she likes.
No matter what you choose to put in the poem for your baby niece, remember to keep it short, simple, personal, and creative, and it will become an heirloom to her.