Modeling agencies can be very helpful to you in your efforts to get your baby involved in modeling. Even if they don't specialize in baby talent, they will have connections to legitimate people who have real work for baby models. Modeling agencies should always be investigated before signing up, but it shouldn't be too hard to find one that has a good track record and professional reputation.
Through searching for modeling agencies who deal with babies and their parents, you will learn a bit about certain services that some modeling agencies provide and different ways of dealing with clients that others use which you will not find with other agencies. Your job is to find on that has the right combination for you and your baby that makes it a better choice for you than all the other modeling agencies.
Some modeling agencies really will go out of their way to accommodate your needs and look out for the best interests of you and your baby, however, there are others that have only their own best interests in mind and will more likely be looking for ways to cheat you out of money, especially at the stage of actually signing up for their services. Modeling agencies sometimes will try to get you to pay for their service upfront by "offering" a head shots that they charge an exorbitant amount of money for - avoid this type of agency always.