depending on whether you'll be out most of the time or not. We used to stay a while at countries with temperature 4-8 degree when dear son was 0-2, ok for him. Anyway, their temperature is a little warmer than adults, shouldnt find any problem. Especially when they start to run about, wont b that cold anyway. Stay away from the wind, should be ok.
Anything below 0, beta not bring them..too cold, even for me...especially if windy.