The answer to your question depends on if the agency is franchised with SAG. In the theatrical world, most agencies aren't but I'm not so sure about commercial companies.
If they are, the contract will be SAG issue with a rider attached. The actual SAG contract is drawn up by the union so you have nothing to worry about. The rider is for signing under aged actors.
If they're not franchised with SAG, then they will present you with a General Services Agreement along with a similar rider. This is ok too but you should read the contract carefully because different agencies format the terms to fit their needs. (Look for the length of the contract and an out.)
I wish I could be more helpful but I'm an adult theatrical agent only. I did make a call on your behalf and a respected kid's agent sent me a copy of his standard rider. I've attached it below. Good luck!
The foregoing is an agency contract between _________________________________,
(hereinafter “Actor”), a minor, and __________________________, the minor’s parent or guardian (hereinafter “Parent/Guardian”) whereby each engages the services of XXX Talent Agency to provide agency services for Actor on the terms and conditions provided therein and under the terms and conditions of the agency contract.
In addition thereto, it is expressly agreed and understood that (a) Actor and Parent/Guardian are each bound by all the terms and conditions of said contract as a party thereto and (b) that Parent/Guardian shall unconditionally guarantee the performance of Actor there under on a continuing guarantee and surety basis.
ACTOR (a minor)
___________________________
PARENT/GUARDIAN
____________________________
AGENT
By _________________________
Date: _______________________