I'd say about 6–8 weeks under heat if they were then going outside into reasonable temperatures. Better done in the spring therefore.
It's a great thing for kids to rear chicks and see the changes, I'm sure you won't regret doing it

I incubate and hatch chicks regularly and love it, they graduate from the kitchen, to the hall, to outside, all still with heat - by the time they're ready to leave the house, I'm ready too

But I wouldn't do it at this time of year.
Edited to remove the iPad's 'improvements'