Pretty sure it depends on the venue you are going to at Chase Field.
I believe that kids 24 months and younger get in free. It wouldn't be a bad idea to check with them though! https://www.websitealive3.com/12218/operator/guest/gDefault_v2.asp?cframe=login&chattype=normal&groupid=12218&websiteid=0&departmentid=0&sessionid_=&iniframe=&ppc_id=&autostart=&proactiveid=&req_router_type=&text2chat_info=&loginname=&loginnamelast=&loginemail=&loginphone=&infocapture_ids=&infocapture_values=&dl=&loginquestion=
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