Keep in mind that this is just off the top of my head, but it should at least give you a quick version to mess around with. It works best if you set a token to be a light source for the fetchling rather than just having them emit light if they're carrying a torch. Click the token, click the Edit Token gear, and go to the Advanced tab. On the right in the "Emits Light" section, check "Has Sight", and set the multiplier to whatever the "sees (this many times) better than humans in low light" value is: 2 for elves, 3 or 4 for dragons, etc. Then divide the distance of the fetchling's darkvision by the multiplier you set and put this value in the Light Radius box. If it's standard low-light vision (2x) enter half the range. For the middle box set it to whatever you want. I usually have 60 darkvision dim at 59 feet, but that's personal preference. Start with the same value as the Light Radius, and make it slightly smaller if you want a dim effect at the edges. Don't check "All players see light" because no one else can see through their darkvision. Assuming regular low-light vision for double range, and 60' darkvision, you end up with 30 ft. Light Radius, 29.5 foot Start of Dim, All Players See Light un checked, and Has Sight checked, with the multiplier set to 2. The multiplier doubles the values you set for Light Radius and Start of Dim, so those become 60 ft. darkvision with a 59 ft. dim radius. It also doubles the range of any other light sources to account for low-light vision. If you drop a torch token onto the map and set it for 40' light radius and 20' start of dim (D&D 3.5 standard: 20' bright light, dim light out for 20' more), and check "All Players See Light", you'll see that your fetchling token sees 40' of bright light and 40' more of dim light from the torch, while maintaining their 60' darkvision. To test what the Fetchling sees, click the token and hit CTRL+L. Hope this helps!
