-Filing MTR is Form I-290B, not I-129B. If your child is too young to sign the form, you can sign her name and sign your name aside, and indicated the signature is on behalf of her. If you have a lawyer to deal with this MTR, the lawyer can sign the form for your daughter and you do not need to sign it.
(2) In this situation, do I need to use a lawyer to help filing the MRT?
-It is up to you, but I would do it by myself because this is very simple procedure.
Good luck!