I had three broodies this spring. I had someone give me 6 meat bird chicks from their order of 50. I didn't really want to brood them, so I gave each momma 2 chicks. I did it at night.
After I gave them the chicks, I snuck back out after about 15 minutes with my flashlight. One of the little chicks was wandering around, cold and crying.

I gave it to one of the hens that seemed to be accepting the 2 I gave her.
I went back out again in about 15 minutes with my flashlight and there was another chick wandering around, cold and crying.

I gave that to the hen that was doing well with hers as well...
In the end, all 6 chicks ended up with the one momma hen. She was a banty and raised her meat birds in a separate pen for about 4 weeks until they were HUGE and didn't seem to need her anymore. It's not like she could keep them warm!! LOL She went back into the main pen and fit right back in with the rest....
So, I guess my advice is, do it while the momma is sleeping and be prepared to check on them several times over the next hour or two with a flashlight and then early in the morning and over the next day. If they make it through that, then they should be fine!
Good luck!