As you say, not outside in this weather.
Look at the chicks behaviour, if they're grouped together, hunched and fluffed up, or making that loud cheep noise, they're far too cold. If they're happily cheering abuot eating, they're fine. At 8 weeks I'd be looking to get my chicks off heat, outside, but depends upon many factors.
Could you put electric hen on a timer plug so it cuts on and off for a few hours in the night so it's just a few hours in the coldest time of night?
If they're cold they will eat more. I give my bantams a heat lamp in all cold weather above their perch, they're adults!
Your chicks are probably tougher than you'd think, if they've got good feathers and can huddle together at night, so as long as they're not 'sad', they'll be fine.