T.F.
I have modeled most of my life. One thing to keep in mind is that it is WORK.
If an agency is interested in you, they will contact you. You spend NO MONEY or you are being scammed. They do not need a portfolio, a simple snapshot is just fine.
Most legit companies do not advertise. You submit a snapshot and they will contact you if you are a good fit for a particular shoot.
IF you are called to a casting... keep in mind, you go when you are told to do so which may bein the middle of nap time, fussy time, dinner time, etc. Be prepared for a lot of rejection because everyone thinks they have the most beautiful baby and it is ruthless out there, especially with some parents.
IF your child does not like it, STOP. Do not do it for you because it is your child working, not you. If they do like it, allocate ALL the monies earned to the child.
I continue to model occasionally. My 17 yr old models occasionally as well. It is not always fun and within your schedule. Be ready for loads of rejection but don't think for a minute it is because your baby is not pretty.. they match models to the jobs.