Came across this on another website, adult fare is $16.50. A good saving over a cab if travelling to/from Airport with kids.
Not sure if a Go Card would know this, may pay to keep this link handy and get a child to 'tag off' at a city station (or not 'tag on' at Airport). Inspectors should be able to verify your travel. Or just check at the station before you travel. I don't think Go cards get a discount on Airtrain anyway so worst case just get a paper Airtrain ticket for that part of the trip and tag in city. I think any station should be able to sell you this.
How much do Kids normally pay ???