You really should look at Travelnet under Pass Policy, as several of the answers here have been incomplete or not in compliance with policy. Search for "accompaniment of children" dated 3/10/15. There are restrictions on the types of flights and listings you can do based on age (5 and 14 are the magic numbers). By policy you're supposed to fill out UM paperwork, but no UM services are provided (no charges either). There are some other rules and restrictions published in the preamble about accompaniment of which you should be aware.