I would recommend the two agencies you have mentioned. I would check their websites for information about how to get represented. I know this is on both sites as we looked into it as well. This is a very demanding "job". They may call you and ask you to be somewhere that afternoon. And, it's hard to say NO because if you do, they may not call you back.
Both of my children were with an agent in Atlanta (Atlanta's Young Faces) and they both did quite a bit of work. My daughter did a lot of modeling... my son actually landed a national commercial! It was a great experience for both of them.
I can tell you to stay away from Paige Parkes agency. They will tell you that you can get him in with Disney and all this other stuff... only to get you to sign up for their "camps". They tried to get $1,500 out of me saying they would represent my kids but in the end that was for a camp to get headshots. The fine print said there was no guarantee they would represent you even if you did the camp. I talked to my agent in Atlanta and she said to RUN away from them. If you talk to ANYONE be sure to read the fine-print. AND, you should not have to pay any up front fees except for headshots. Fees are only taken out when money is made.
My kids were with the agency for 3 years so I know a lot about the industry. If you have any questions, please don't hesitate to send me a private message and I will give you my email address.