My group is all under 2.5 and my cutoff is -15 with the wind chill. Colder than that I have in my policy that I will use my discretion to determine if and for how long we go outside. We have spent a lot of time outside this winter and the kids are definitely feeling cooped up (or maybe it's just me?).
I don't really have it as part of my official handbook but I send out an email when the cold weather starts to remind my clients.